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Thread: Brakes - Operation of the Master Cylinder

  1. #1
    John Heath jfheath's Avatar
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    Brakes - Operation of the Master Cylinder

    Some basic information here about the operation of the brake master cylinder.

    This is a working document. Please fell free to quiz, correct, add to anything in here. It's basic stuff, but in helping a few friends recently I realised that they were at the stage that I was at when I didn't know it either ! I hope it helps.

    This document was last edited on 16 July 2017 in response to comments made before post #9 are shown in grey text.
    And edited again on 17 July 2107 to remove references to 'relief port' and use the more correct 'compensating port'.

    1 - 'Relaxed' Brake Master Cylinder (ie Brakes are not Applied)



    The image shows the front master cylinder and the reservoir immediately above it - just like the ones used on the clutch and brake lever on the ST1300. The piston and push rod are shown in grey.

    The primary seal (blue) is the one that applies considerable pressure to the braking system.

    The secondary seal (black) maintains a body of fluid behind the primary seal which is gravity fed from the reservoir above. It also isolates the system from the outside world. This seal is never normally put under any great (ie braking force) pressure. The ST1300s secondary master cylinder being an exception to this arrangement.

    Note the larger inlet port between the reservoir and the piston/cylinder. This maintains a volume of fluid between the two seals, fed constantly from the reservoir. The inlet port is never closed off by the position of the piston and seals.

    The tiny compensating port is immediately in front (just to the left) of the primary seal. This allows fluid in the braking system to return into the reservoir and relieves any build up of pressure when the braking system is in this 'relaxed' state.

    The spring is responsible for returning the piston to the right on this diagram. The push rod at the right hand end is operated by the brake lever.

    The hose to the brake calliper is shown descending beyond the bottom of the picture on the left of the master cylinder.

    ----------------------------------------------------------------------

    2 - Master Cylinder when Brake is Applied



    In Diagram 2, the piston has been pushed to the left. Fluid under pressure (pink) is forced down the brake line to the pistons in the brake calliper. Note that the primary seal (blue) has moved past the compensating port, preventing any fluid from returning to the reservoir. The fluid under pressure is responsible for pushing out the pistons in the brake calliper which push the brake pads onto the disc rotors. Once the brake pads are in contact with the disc rotors it takes only a small movement in the master cylinder (brake lever) to exert a force sufficient to bring the bike to a halt.

    ----------------------------------------------------------------------

    3 - Releasing the Brake Lever



    When the brake lever is released, the piston and plunger are returned to the initial 'relaxed' position primarily as a result of the spring pressure in the master cylinder, but aided by the flexible hoses and the calliper piston seals returning to their normal state.

    Each time the brakes are applied, the pads wear down a tiny fraction, and the calliper pistons are pushed out further than they were before the brakes were applied. When the brakes are released, the spring returns the master cylinder piston to its relaxed state and a negative pressure (vacuum) is created in the brake lines. Fluid is able to flow past the lips of the primary seal to allow for the fact that the pads have worn down a little.

    The diagram shows the piston towards the end of its travel back to its 'relaxed' state, and the red arrow indicates the flow of extra fluid from the yellow reservoir fluid and into the brake lines (green fluid).


    ----------------------------------------------------------------------

    4 - Dealing with Pressure Build-up



    When the brake is not being applied, and the system is in a relaxed state, it is important that fluid is able to 'flow' to and from the reservoir as required. This is can happen when:-

    * A slight warps in the disc rotors or general chattering of the brakes over bumpy surfaces pushes the pistons in slightly.
    * An increases in temperature causes the fluid to expand.
    * The bike cools down in the garage overnight and the fluid contracts. The relief port allows fluid to flow into the brake lines rather than suck the calliper pistons back.
    * The brake pads are replaced and the calliper pistons are pushed in. The displaced fluid returns to the reservoir through the tiny relief port. No damage can result in pushing in the calliper pistons in this way if the system has been flushed with new fluid and the exposed calliper pistons are clean. Otherwise it is better to expel the old fluid from the calliper bores via the bleed valve.

    The picture shows the pressure in the brake line being allowed to pass through the tiny compensating port into the reservoir, once the brake lever has been released. To prevent the 'fountain' shown in the diagram, a small chromed disc clips slightly above the port at the bottom of the reservoir.

    ----------------------------------------------------------------------

    5 - Blocked Pressure Relief Port



    If the piston is prevented from returning to its proper 'relaxed' state, then the Primary Seal will not expose the tiny compensating port. This could be caused by corrosion behind the piston, as shown. This part of the piston is open to the elements unless treated with silicone grease and covered with a rubber boot.
    Alternatively, the compensating port itself could be blocked. Fluid that isn't replaced every year can turn into a thick gel which accumulates in the bottom of the reservoir. Or perhaps debris has fallen into the reservoir during a service.

    Whatever, if the compensating port is not clear, or the piston fails to return properly to expose the compensating port, pressure builds up (red) and the brakes lock on solid. I have seen one situation recently where a master cylinder service kit was supplied incorrectly. The push rod was slightly too long and front brakes locked on solid at the first application and would not release.

    ----------------------------------------------------------------------

    The illustrations refer to the ST1300 front master cylinder, but the principles also apply to the secondary master cylinder and the rear master cylinder, even though their relief port is connected to the fluid inlet line rather than to the reservoir itself.

    -----------------------------------------------------------------------
    Final note. Throughout this document, I have made much more of the role of the compensating port in relieving pressure than its other functions.

    In fact, the port will allow fluid to flow in either direction to add or remove fluid from the lines to compensate for the fact that the existing fluid will expand or contract as the temperature fluctuates. To do this, the master cylinder piston has to be returned to its relaxed position so that the port is not blocked by the seal. The spring in the master cylinder ensures that this happens.
    Last edited by jfheath; 07-17-2017 at 12:55 AM.

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  3. #2

    Re: Brakes - Operation of the Master Cylinder

    Quote Originally Posted by jfheath View Post
    Some basic information here about the operation of the brake master cylinder.

    ----------------------------------------------------------------------

    3 - Releasing the Brake Lever



    When the brake lever is released, the spring pushes the piston and plunger back to its initial 'relaxed' position. The diagram shows the piston at the start of its return to the 'relaxed' state. The fluid in the brake line (green) is no longer under pressure. As the brake pads wear down, the pistons in the calliper will need to push out further and this requires more fluid. As the MC piston is pushed to the right, by the spring, the fluid behind the primary seal (yellow) is able to flow past the outer edge of the seal to take up the space created by the extended pistons.

    ----------------------------------------------------------------------
    Thanks for putting this together!

    A quick comment on (3). The piston is shown in the same position as in (2), so although the lever is released, the pressure in the line is still the same as in (2). There is no back flow across the front seal as the line pressure is higher than the reservoir pressure and the seal is still energized (remains energized until it travels back across the port). The spring does push the piston back, but initially the pressure (up to about 2000 psi) does most of the work. The spring is essential to insure the seal completely clears the port once the seal reaches the port and the line pressure equalizes with the reservoir pressure and there is no pressure differential left across the seal to drive the piston back. And then the spring insures the piston stays back, which is critical as illustrated by your subsequent schematics.

  4. #3
    John Heath
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    Re: Brakes - Operation of the Master Cylinder

    Thanks for the comments - much appreciated.

    Quote Originally Posted by MidLife View Post
    The piston is shown in the same position as in (2),
    So it is. Oops - I had intended it to be shown part way between position 2 and the 'relaxed' position. Text and Pic now edited (here and in original)

    Quote Originally Posted by MidLife View Post
    There is no back flow across the front seal as the line pressure is higher than the reservoir pressure and the seal is still energized (remains energized until it travels back across the port). The spring does push the piston back, but initially the pressure (up to about 2000 psi) does most of the work.
    Ok - I understand that there can be no flow across the seal if there is pressure in the system - I'm trying to simplify things to make the process easier to understand, without actually being wrong !

    The returning pressure when the lever is released comes from the calliper piston seals returning from their deformed state and from the limited compressibility of the fluid itself. As the pads wear down, the calliper pistons will move back less then the amount that they moved to apply the brakes - a miniscule difference each time, but the spring returns the master cylinder piston anyway. In doing so, at the end of its return, fluid is able to pass the primary seal to compensate for the fact that the calliper pistons are now further out than before.

    Or are you saying that fluid never flows past the primary seal and that the fluid is replenished via the relief port?
    Last edited by jfheath; 07-02-2017 at 12:48 PM.

  5. #4

    Re: Brakes - Operation of the Master Cylinder

    Quote Originally Posted by jfheath View Post

    The returning pressure when the lever is released comes from the calliper piston seals returning from their deformed state and from the limited compressibility of the fluid itself.
    When braking, the line pressure starts building up as soon as the front seal passes the compensating port. With no leak, whatever pressure was built up while braking will push the piston back. As you say, pad wear for a single brake application will not cause noticeable oil volume change, the heat from hard braking however can cause fluid expansion which will raise the pressure. Volume expansion is made up for once returning over the compensating port.

    Line expansion, as you know, also plays a role in how much travel it takes for building up pressure (the relatively soft lines act as an accumulator bladder), and can be reduced by installing stiffer reinforced lines.

    Fluid compressibility is limited but far from negligible. Especially considering that even brand new fluid already contains some air and air content increases with usage.

    I suppose you have taken a ST 1300 master cylinder apart? Am curious about the orientation of the secondary seal. In most applications, such a seal would be oriented in the opposite direction of the primary seal (opposite of shown in the schematic), to keep the crud out. At times, even a different type seal is used as secondary.


    Quote Originally Posted by jfheath View Post

    Or are you saying that fluid never flows past the primary seal and that the fluid is replenished via the relief port?
    The "relief port" is often called "compensating port", as it allows oil to flow both ways to compensate for volume changes due to temperature changes for instance and other effects that you have described.

    I can of course not say that fluid will "never" flows past the primary seal, as I know too little about the system.

    I can only say that in normal circumstances, I see no obvious reason for the fluid to flow over the seal.

    Take the case where you grab the brake while standing still: Whatever is compressed in will expand back out when releasing the lever and there is no need for flow by.

    On a single application while riding, the volume change from pad wear would be so small that it would not warrant flow by either.

    On a longer ride with frequent applications, the reservoir can be viewed as acting similarly to the coolant reservoir. Oil will expand in the reservoir through the compensating port as it heats up and then flow back into the system as it cools down after the ride, back down to a level where it compensates for all the wear that occurred during the ride.

    One possible case where oil would need to flow by the seal could be if there is leakage in the system. The spring would push the piston back when releasing the lever and create a slight vacuum in the line which would call for flow by to allow the piston to travel all the way back (which could explain why the hole between the two seals is larger, to prevent too restrictive of a flow in case flow back is required to compensate for a leak. This would allow quick pumping of the brake to overcome the leak and come to a stop).
    Last edited by MidLife; 07-02-2017 at 03:01 PM.

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    John Heath
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    All good points, MidLife - thanks for taking the time to contribute.

    Yes the seal cups both face in the same direction. The photos in the workshop manual show this as well. The manual makes a point of making sure that the cups are not turned inside out when replacing the piston and seals into the master cylinder bore. The seals are of slightly different design, but both are cups.

    I thought long and hard about how fresh fluid is pumped into the brake lines and the calliper piston bores when the brake lever is operated. Some questions I wanted answering:

    Why does the inlet port feed fluid in between the primary and secondary seal ?
    How does the fluid then get into the brake lines ?
    What happens when I push the pistons back into the callipers ?
    When I press the brake pedal, how come fluid comes out of the outer bleed valve on the rear calliper ?

    I thought about what happens when the callipers are being cleaned - the callipers are not bolted to the bracket, the pads aren't in. Once the pistons are clean, I push them back into the callipers. There is a fair bit of resistance, but slowly they can be pushed home under thumb pressure. The fluid is being pushed back into the reservoir. Take out the little chromed cap that sits over the compensation port at the base of the reservoir, and the jet of fluid is obvious. The compensation port is tiny. I cleaned one out with a primus stove 'pricker' - the only thing I could find that would fit. 0.26mm (.01")

    However, when I pump the pistons back out again, I can squeeze the lever - the pistons move out a noticeable distance. Release the lever, the pistons do not move back in, but the brake lever is ready to pump another load of fluid down the line to move the pistons out further. There is no need to wait, the brake lever is ready as soon as it has been released. This is much faster than would be possible if the fluid was being replenished only through the compensation port. Remember how long it took to push the calliper pistons in.

    So I reckoned that the master cylinder piston and primary seal were acting in a similar manner to a bicycle pump which forces air forward on the compression stroke, and allows air to pass the seal on the return stroke. This behaviour in the master cylinder would explain why the inlet port feeds fluid into the system behind the primary seal.

    I checked this out on-line. I saw three different methods of getting fresh fluid into the brake lines. In one, the inlet port was positioned in front of the primary seal (where our compensation port is located). Fresh fluid can flow into the system when the master cylinder is relaxed. The inlet port is closed off when application of the brakes pushes the seal past the inlet port. There was no compensation port in this method. In another example, the piston itself is metal with a seal around the outer edge (like the seal on the calliper pistons). But the metal piston has holes in it with a one-way valve (like a reed valve). On compression the valve closes. On the return stroke, the valve opens and fluid is able to flow through it if necessary. The third type allows the fluid to flow past the primary seal cup on the return stroke.

    I suspect that what you say is what happens under normal braking circumstances. Given the miniscule amount of wear on the brake pads at each application, there will be need only for a very tiny amount of fluid to fill the space created by the tiny extension of the calliper piston. As such, there would probably be insufficient flow to squeeze past the primary cup, and the fluid may well be replenished as the seal passes the compensation port under spring pressure.

    I was watching British Superbikes yesterday. One of the commentators commented on a close up of a rider who seemed to be applying his front brakes as he was accelerating out of a bend. The other commentator said that they have to do that - he got a bit of steering headshake as he pulled away, and that pushes the pistons back into the callipers a bit. He has to pump them up again ready for the next braking point. You only make that mistake once in your career !

    Your insight has been extremely useful - you are describing what happens under normal braking circumstances and I have been looking at what happens when the bike is being serviced in the garage - and I have a reason for taking that approach, but recognise that the picture I have painted is only the 'gist' of what actually happens. Perhaps I should have said why I put this little document together. I started working on a different document that described ways of recognising when the braking system was just beginning to 'play up'. Particularly with respect to the SMC. In doing so, I kept tripping over myself because I was finding it necessary to describe what was happening inside the master cylinder - so I thought I'd get that out of the way first !

    With that in mind, the aim was to give just an overview of what I understand is going on in the master cylinder. So for example, I deliberately chose the term 'relief port' because in the situations that I want to mention, it is the relief of pressure from the system (or the inability to relieve pressure) that is crucial. But clearly the port works in both direction compensating for difference in fluid volume in either direction. I also wanted to get across the idea that the primary seal seems to act like the reed valve, allowing fluid to flow past it from behind - because that appears to happen in the SMC when the rear brake pedal is pressed (in the garage) to get fluid to the outer two pistons of the rear calliper.

    I think your contribution has filled in the numerous gaps that I had left in an attempt to keep things simple. All useful stuff. Thanks.
    Last edited by jfheath; 07-03-2017 at 01:57 AM.

  7. #6

    Re: Brakes - Operation of the Master Cylinder

    Quote Originally Posted by jfheath View Post

    Your insight has been useful
    More like trying to keep up with all the thoughts that lead to your instructive article.

    Looking forward to your upcoming SMC article.

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    Re: Brakes - Operation of the Master Cylinder

    Hello jfheath,

    I see you've been busy studying the St1300 brake system, it is an interesting system!

    I attempted to comment on your post, but I may have dawdled too long, as I can't seem to Quote from the original. No worries I copied & saved it!

    1 - 'Relaxed' Brake Master Cylinder (ie Brakes are not Applied)
    The primary seal (blue) is the one that applies considerable pressure to the braking system. The secondary seal (black) prevents leakage to the outside and water / air ingress. This seal is never put under any great pressure.
    I believe this to be in error: The secondary seal is a component of the recuperation phase of master cylinder operation, it is not a weather seal.

    Note the larger inlet port between the reservoir and the piston/cylinder. This keeps the space between the two seals full of fluid and is a gravity feed. The tiny relief port is immediately in front (just to the left) of the primary seal. This allows fluid in the braking system to return into the reservoir and prevents a build up of pressure when the brake is in this 'relaxed' state.
    Identifying these two inlet ports can get confusing very quickly. The smaller port is commonly named: Bypass port, Intake & Return port, and Compensating port. Where as the larger port is commonly named: Intake port, Equalization port, Feed port, Replenishing port, and Recuperation port. In an attempt to reduce confusion, I've tried to use the terms Compensating port, and Recuperation port exclusively, when discussing the operation of Honda master cylinders.

    3 - Releasing the Brake Lever
    When the brake lever is released, the piston and plunger are returned to the initial 'relaxed' position. Initially this is from residual fluid pressure and from the residual energy stored by the deformation of the piston seals. Each time the brakes are applied, the pads wear down a tiny fraction, and the pistons may remain further out than they were before. The spring ensures that the master cylinder piston is returned all of the way so that the relief port is cleared - and that it stays there until the brakes are applied again.
    The diagram shows the piston towards the end of its travel back to its 'relaxed' state. As the spring takes over, the fluid in the brake line (green) is will no longer be under any pressure, and fluid from the right (yellow) can flow past the outer edge of the primary seal to take up the space created by the extended calliper pistons.
    The spring can (and does) push the master cylinder piston back faster than brake fluid can return from the caliper(s), which is due to numerous fluid resistances in the brake system (small internal bore of hydraulic piping, tiny hydraulic ports, delay valve, proportional valve). The effect of the master cylinder piston moving quickly is that fluid pressure drops in the chamber, creating a negative pressure (vacuum), the vacuum draws fluid around the primary seal from the chamber between the primary seal, and the secondary seal (this is the recuperation phase). Without the recuperation elements in place the vacuum would draw in air, and other contaminates from the outside.

    4 - Dealing with Pressure Build-up
    * The brake pads are replaced and the calliper pistons are pushed in. The displaced fluid returns to the reservoir through the tiny relief port.
    This probably is not the best way to re-position caliper pistons when replacing brake pads. The caliper (usually) is the lowest component in the hydraulic system, over time debris and contaminates in the fluid will have settled in the caliper, stirring that sediment up by pushing in the pistons, and then forcing the dirty fluid back towards the master cylinder seems undesirable. A better method is to open the bleed screw, wasting the brake fluid as the caliper pistons are re-positioned. Then topping up the master cylinder with fresh brake fluid.

    5 - Blocked Pressure Relief Port
    If the piston is prevented from returning to its proper 'relaxed' state, then the Primary Seal will not expose the tiny relief port. This could be caused by corrosion behind the piston, as shown. This part of the piston is open to the elements unless treated with silicone grease and covered with a rubber boot.
    This part of Honda master cylinder design appears to be the 'Achilles Heel' for the ST1300. Each master cylinder (there are three) provides minimal protection from the elements with merely a rubber bootie covering the open end of the master cylinder bore. Amazingly the rubber bootie seems to be adequate for both the hand & foot operated master cylinders, with only the occasional complaint of problems. However the location & positioning of the SMC (secondary master cylinder) serves to high-light just how woefully inadequate the rubber bootie is in protecting this master cylinder from the elements, as evidenced by the large quantity of complaints of corrosion in the SMC bore.
    Last edited by Anna'sDad; 07-15-2017 at 01:03 PM. Reason: Oops, Got myself turned around re primary/secondary

  9. #8
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    Re: Brakes - Operation of the Master Cylinder

    Hello MidLife,

    I was reading your comments re jfheath's tome on ST1300 brakes, when a couple of items caught my attention.

    There is no back flow across the front seal as the line pressure is higher than the reservoir pressure and the seal is still energized (remains energized until it travels back across the port).
    Sorry, I believe this is not correct; due to the systemic resistances to fluid flow, the master cylinder piston spring will move the piston faster than the flow of returning fluid from the caliper can respond. This action will create a negative pressure in the master cylinder bore, this difference in pressure will force fluid to flow from the recuperation port, slipping past the primary seal, and filling the area between primary & secondary seals. When the master cylinder piston has returned to it's resting position, and reveals the compensating port, the slow returning brake fluid (as well as the 'additional' fluid that slipped past the primary seal) will return to the reservoir by way of the compensating port.


    The spring does push the piston back, but initially the pressure (up to about 2000 psi) does most of the work.
    The brake system pressure rises after all brake caliper piston motion has essentially stopped, continued application of the brake lever attempts to compress the fluid, which causes system pressure to increase, in turn applying the brake pads to the brake rotor. At this point it takes very little movement of the master cylinder piston to generate the high fluid pressures necessary to bring the vehicle to a stop. Inversely, when the brake lever is released the high system fluid pressures diminish rapidly before any appreciable movement of the master cylinder piston is noted, this lack of sustained high fluid pressure causes the flow rate of returning caliper fluid to slow substantially.

    The spring is essential to insure the seal completely clears the port once the seal reaches the port and the line pressure equalizes with the reservoir pressure and there is no pressure differential left across the seal to drive the piston back.
    Agreed, however the pressure differential is a negative pressure in the master cylinder bore, with atmospheric pressure on the reservoir the greater of the two.

    Over the years of my ST1300 ownership, I've disassembled most of the ST1300 brake components (some of which will never go back together again) in my quest to fully comprehend how this system was designed to operate. I have also generated a number of sketches of the various components to aid in understanding this brake system, most of those drawings are available here on st-owners.com for anyone to view.

    Cheers, Anna'sDad
    Last edited by Anna'sDad; 07-16-2017 at 04:02 PM. Reason: Oops, got my primaries & secondaries wrong way round.

  10. #9
    John Heath
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    Re: Brakes - Operation of the Master Cylinder

    Thanks for the comment David. I'm wondering if one of us has the terms 'Primary Seal' and 'Secondary Seal' the wrong way round ?

    PM Sent.

  11. #10
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    Re: Brakes - Operation of the Master Cylinder

    I'm wondering if one of us has the terms 'Primary Seal' and 'Secondary Seal' the wrong way round ?
    That would be me.

    Seems I got my primaries & secondaries wrong way round in my response to both your 3 - Releasing the Brake Lever topic, and Mr. Midife's There is no back flow across the front seal as ... topic as well.

    I've made the appropriate corrections where noted.

    Thanks for keeping me on my toes.

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