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Thread: Brakes - Operation of the Master Cylinder

  1. #11

    Re: Brakes - Operation of the Master Cylinder

    Quote Originally Posted by Anna'sDad View Post
    Hello MidLife

    The brake system pressure rises after all brake caliper piston motion has essentially stopped,

    Cheers, Anna'sDad
    If the brakes function properly, there is practically no brake piston motion and hence no oil flow. The master essentially just increases the pressure in the line.

  2. #12
    John Heath
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    Re: Brakes - Operation of the Master Cylinder

    Quote Originally Posted by MidLife View Post
    If the brakes function properly, there is practically no brake piston motion and hence no oil flow. The master essentially just increases the pressure in the line.
    I can understand what you are saying, but I have to disagree. It's beyond the purpose of the article that I wrote, in which I was trying to keep things simple. However, your point is worth discussing.


    I wondered about this same issue, but explained it to myself:

    When I put new brake pads in, the pistons are almost flush with the calliper body. When I replace them a few thousand miles later, the pistons extend approximately 1cm from the calliper body. (Assuming that both of the pads have worn down by 5mm).

    In order that the pistons are able to move by this amount, a considerable amount of fluid has to pass from the fluid reservoir into the brake lines. The only way that it can do this is by passing the outside lips of the primary seal when the piston is on its return stroke. The fluid that is between the primary and secondary seal of the master cylinder piston is constantly fed from the reservoir via the inlet port.

    The behaviour can be observed when pumping fresh fluid through the system with the brake lever. The flow of fluid through the system is far too fast for the fluid to be replaced via the compensating port - which is not entirely shut off for the pumping cycle - except briefly when the lever is released. It has to flow past the primary seal.

    Of course, when the brakes are applied, the amount of movement involved at the calliper piston is quite small. The brake pads lie very close to the disk surface, but they still need to be moved in order to press hard against the disc rotor. The brake friction material wears down a small amount each time the brakes are applied. Admittedly this is only a very small amount, but it is not zero.

    I don't think I had considered this before, but I think Anna'sDad's comment is spot on in describing the negative pressure that is created as the master cylinder piston is pushed back by the spring. There is no way that the deformed seals, the calliper pistons the brake lines and the fluid can return to their normal state as fast as the spring can return the master cylinder piston. It creates a negative pressure in the brake lines and this allows fluid to flow past the primary seal in the master cylinder. (In effect, the negative pressure is sucking fluid past the seal in the master cylinder). As Anna'sDad says, once the seals, fluid, pistons and brake lines have returned to normal, the compensation port is open and the pressures in the system are normalised.

    There is another consideration in all of this - The master cylinder is quite small in diameter - say about 12mm diameter. The calliper outer pistons are (say) 24mm - twice the diameter, so 4 times the surface area. There is only one master cylinder piston, but there are 4 front brake calliper pistons which are serviced by the same master cylinder. This means that the calliper piston movement is only 1/16th of the master cylinder piston movement. It is this reduction ratio that allows the brakes to be applied with such force from a handlebar lever.

    To put figures on this, for every 0.1 mm of friction material worn off the pads, the calliper pistons have to move 0.2mm (pads wear evenly on both sides). This would require 3.2mm of movement at the master cylinder piston. The amount of fluid flow in the master cylinder starts to look a lot more significant ! (Assuming 10,000 miles for brake pads, 0.1mm would represent the amount of wear in 200 miles).

    PM me if you want the mathematics for this.

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

    It may be that when the pistons move to apply the brakes, the pistons do not always move past the seals. I imagine that for some applications of the brake the seal stays 'attached' to the piston and the seal flexes to allow this tiny movement of the piston to take place. When the brake lever is released, the seals effectively draw the piston back to the same position that it was before, but now the brake pads will be slightly further away from the disk surface - as a tiny amount of friction material has been worn away from the pads. In this case there is no net change in the amount of fluid in the system once the system has stabilised.

    If the above is true (and I am only hypothesising), it cannot always be the case, as clearly the pistons have to move out as the pads wear down. So after a few more applications of the brakes, the pads will have worn down significantly more. Now when the brakes are applied, the movement required from the pistons will be greater than the amount by which the seal can deform, and the pistons will break away from the seal and slide past by a small amount. When the seal returns to normal, the piston will move with it, but because it slid past the seal the piston will remain further out than it was before. The piston has adjusted itself to bring the pads closer to the disk surface again. The calliper piston bores now accomodate more fluid than before and as the master cylinder piston returns it creates a larger negative pressure than before and the flow of fluid past the primary seal is much greater as the master cylinder piston returns.

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

    Whatever the truth in that last section, the key point is that when the brake lever is released, the master cylinder piston springs back to its relaxed position. In doing so, the fluid in the lines and the calliper cannot keep up with the speed of the master piston. The primary seal yields to allow fluid behind it to pass to the front. It doesn't matter how small the amount of fluid, the difference in pressure allows it to happen.

    If you are not convinced that fluid can pass the primary seal from behind, try bleeding your rear calliper outer pistons with the brake pedal. Fluid flows easily, but it is coming through the SMC from behind the primary seal.

  3. #13

    Re: Brakes - Operation of the Master Cylinder

    Quote Originally Posted by jfheath View Post
    To put figures on this, for every 0.1 mm of friction material worn off the pads, the calliper pistons have to move 0.2mm (pads wear evenly on both sides). This would require 3.2mm of movement at the master cylinder piston. The amount of fluid flow in the master cylinder starts to look a lot more significant ! (Assuming 10,000 miles for brake pads, 0.1mm would represent the amount of wear in 200 miles).

    PM me if you want the mathematics for this.

    My simplistic approach:

    By the time the pads are worn out, the fluid in the reservoir will have dropped by let's say about 2 cu-in.

    Let's also say that it takes about 10'000 brake applications to get there: this equate to an average of only .03 mm of master cylinder piston motion per brake application to pump this volume in the caliper pistons (using the 12 mm figure for the master piston diameter that you indicated above). Compensating for this .03 mm on the return stroke will hardly require any oil flowing around the primary.

    Even an enthusiastic rider wearing the pads after 1000 applications would only cause the piston motion to travel an extra .3 mm to make up for wear.



    Quote Originally Posted by jfheath View Post

    If you are not convinced that fluid can pass the primary seal from behind......

    You convinced me. No doubt the fluid has to be able to pass the seal, otherwise the Grand Prix riders you mentioned would not be able to pump the caliper pistons back out when needed and a slow leaking worn out older primary seal would not be able to pop right back when the lever is released.



    Quote Originally Posted by jfheath View Post

    Whatever the truth in that last section, the key point is that when the brake lever is released, the master cylinder piston springs back to its relaxed position. In doing so, the fluid in the lines and the calliper cannot keep up with the speed of the master piston.....

    Sure.

    But again, in normal braking conditions, how many of us just let go of the lever?

    And then, when compressing the oil to apply the brakes, the response of the front feels pretty darn instantaneous. Not sure what would cause a delay on the decompression cycle.

    One day, if one of us has nothing better to do, we could try assembling a cylinder without the spring, to get a feel for how noticeable this oil decompression delay might be.

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