ST1300 Rear Brake Problems

[Igofar]

I don't. I put it back after I bleed the PCV. Then I continue with the rest of the rear brake bleed.

Shame on you Mellow
Your suppose to leave the SMC bracket tilted for the process right up until you have to re-mount it to bleed the lower left bleeder.
Top Left, Top Right, Tilt caliper, PCV valve, Center, lower right front, REMOUNT caliper - bleed lower left front, last one on the rear.
You may be moving trapped air from one area in the circut to another, then missing it by remounting it too soon.
See your service manual.
Igofar

 

[Mellow]

Shame on you Mellow
Your suppose to leave the SMC bracket tilted for the process right up until you have to re-mount it to bleed the lower left bleeder.
Top Left, Top Right, Tilt caliper, PCV valve, Center, lower right front, REMOUNT caliper - bleed lower left front, last one on the rear.
You may be moving trapped air from one area in the circut to another, then missing it by remounting it too soon.
See your service manual.
Igofar

Never had a problem with my rear brakes.

 

[Igofar]

Yeah but you still use a vac......dang I almost said it....too.
Hope you and the wife are doing well.
Larry

 

[Anna'sDad]

how does the rear pedal push all three pistons on the rear caliper out when the bike is at a standstill and the SMC is not being utilised

I'm not sure I've read this correctly. Are you asking; when the motorcycle is at rest (perhaps on the center-stand), and the rear brake pedal is depressed do all three pistons in the rear caliper react?

This information is incorrect, see this post for updated information.The answer is no; if the SMC (secondary master cylinder) is operating properly. However my experiences with a corroded SMC is that yes it is possible for the rear brake pedal to unintentionally cause all three pistons (of the rear brake caliper) to react.

I've made a sketch of the SMC that I rebuilt.

When the SMC piston is prevented from fully retracting, the primary seal blocks the compensating port, which in turn prevents brake fluid from passing back to the 'supply' side of the hydraulic circuit. With brake fluid 'trapped' between the SMC and the rear brake caliper, and no where to go as the brake fluid expands, the rear brake caliper pistons begin to creep out causing the rear brake pads to drag.

So what about the two outside rear brake caliper pistons?

Well, imagine a motorcycle that has sat all night with a rear wheel that can spin, and a corroded SMC. What happens when the brake pedal is applied for the first time?

• The rear brake caliper receives pressurized brake fluid causing the center piston to activate as usual, and indeed the center piston will retract as usual when brake pedal pressure is removed.
• The SMC also receives pressurized brake fluid, but due to the compensating port being blocked, the pressurized brake fluid that leaks past the primary seal causes the two outside pistons of the rear brake caliper to activate. Unfortunately (again due to the blocked compensating port), when brake pedal pressure is removed the two outside rear brake caliper pistons do not retract, but instead remain activated causing the rear brake pads to drag.

My experiences with the SMC on my motorcycle leads me to believe that while this is a very critical component of the hydraulic brake system, it is also very sensitive to the effects of it's environment, requiring a greater amount of care and maintenance then other hydraulic brake systems I've encountered.

 

[alienskydiver]

I'm not sure I've read this correctly. Are you asking; when the motorcycle is at rest (perhaps on the center-stand), and the rear brake pedal is depressed do all three pistons in the rear caliper react?

The answer is no; if the SMC (secondary master cylinder) is operating properly. However my experiences with a corroded SMC is that yes it is possible for the rear brake pedal to unintentionally cause all three pistons (of the rear brake caliper) to react. I've made a sketch of the SMC that I rebuilt.

When the SMC piston is prevented from fully retracting, the primary seal blocks the compensating port, which in turn prevents brake fluid from passing back to the 'supply' side of the hydraulic circuit. With brake fluid 'trapped' between the SMC and the rear brake caliper, and no where to go as the brake fluid expands, the rear brake caliper pistons begin to creep out causing the rear brake pads to drag.

So what about the two outside rear brake caliper pistons?

Well, imagine a motorcycle that has sat all night with a rear wheel that can spin, and a corroded SMC. What happens when the brake pedal is applied for the first time?

• The rear brake caliper receives pressurized brake fluid causing the center piston to activate as usual, and indeed the center piston will retract as usual when brake pedal pressure is removed.
• The SMC also receives pressurized brake fluid, but due to the compensating port being blocked, the pressurized brake fluid that leaks past the primary seal causes the two outside pistons of the rear brake caliper to activate. Unfortunately (again due to the blocked compensating port), when brake pedal pressure is removed the two outside rear brake caliper pistons do not retract, but instead remain activated causing the rear brake pads to drag.

My experiences with the SMC on my motorcycle leads me to believe that while this is a very critical component of the hydraulic brake system, it is also very sensitive to the effects of it's environment, requiring a greater amount of care and maintenance then other hydraulic brake systems I've encountered.

Yes when at standstill on the centrestand . All three pistons push out using the rear pedal so from what you say even though I can push the caliper up and compressing the smc and it returns ..... it may not be returning enough because of corrosion ?
Guess I had better order a new smc

 

[Anna'sDad]

it may not be returning enough because of corrosion ?

I first became aware of my SMC failure due to an annoying rattle sound coming from the vicinity of the LH front brake caliper when ever I traversed a speed bump. In searching for the rattle I discovered a bit of motion in the SMC, on the order of a millimeter or two, when the SMC was at rest. Only later when I disassembled the SMC did I realize that the play in the SMC motion was caused by the piston not fully returning in the cylinder due to corrosion.

At the time I disassembled the SMC I was ill-prepared in that I had neither a replacement SMC or a rebuild kit. I simply cleaned up all the components, used a bit of scotch-brite to polish the cylinder bore, and reassembled everything. That rebuilt SMC remained in service for almost 50,000 miles before again requiring service.

I currently have a replacement SMC in service, with the original SMC recently rebuilt (this time with the Honda kit) sitting on a shelf. I think the SMC is not one of Honda's better designs, especially in the area of preventing contamination of the cylinder.

 

[RONST1300]

+ This is a big red flag that you're trying to bleed the brakes as if they were a conventional, un-linked system, which isn't the case on the ST.

There are a number of posts on how to do it correctly if you do a search. Also if you get in a jam, Larry is the guy you can call, to walk you through it. It's not simple.
Ron

 

[Blrfl]

I think the SMC is not one of Honda's better designs, especially in the area of preventing contamination of the cylinder.

The 2008-and-later version has better drainage to help that problem.

--Mark

 

[Throttlejockey]

I'm sure this has been covered before...but can you install the 08- version on the earlier bikes?

 

[Mellow]

I'm sure this has been covered before...but can you install the 08- version on the earlier bikes?

I don't believe there's a reason it wouldn't work. But, I haven't measured if the brackets that attach to the forks are the same distance appart... next time I have a pre 2008 bike in the garage for a tire change - maybe this week - I'll measure.

You would have to change both front calipers as the front calipers and pads are larger starting in 2008.

I don't think the smc itself would fit a pre-2008 bike but I'd have to do some more measuring and let you know. Off the top of my head, I just don't think that is an option as I think it's been discussed.

 

[Blrfl]

Most of the brake parts on 2008s and later have a different supplier code (R0) in the part numbers, which suggests that the brakes on those bikes may be different in design than 2002-2007 models. The new SMC bracket may physically fit, but there's always a chance that the hydraulic parts inside behave differently.

The SMC bracket on 2006 and earlier bikes, 06454-MCS-G01, was superseded by a new version, 06454-MCS-G02, in 2007. I haven't seen the 'G02 part in the flesh, but since they revised it, I'd bet the new bracket has the same drainage improvements in the 2008 part.

--Mark

 

[alienskydiver]

So does that mean that if all 3 pistons move as a result of operating the rear brake pedal we have a good indication that the SMC needs repair or replacement ?

Good question

 

[Blrfl]

So does that mean that if all 3 pistons move as a result of operating the rear brake pedal we have a good indication that the SMC needs repair or replacement ?

That would be a good indication that the SMC is allowing pressure from the rear master cylinder (which feeds it) through and is transferring it to the rear caliper.

--Mark

 

[Igofar]

That would be a good indication that the SMC is allowing pressure from the rear master cylinder (which feeds it) through and is transferring it to the rear caliper.

--Mark

 

[Anna'sDad]

So does that mean that if all 3 pistons move as a result of operating the rear brake pedal we have a good indication that the SMC needs repair or replacement ?

That's an excellent question John, indeed one that I've been slowly working my way around to.

All that I've read re the Honda DCBS indicates that the rear master cylinder activates the center piston of the rear brake caliper, and not the two non-center pistons. However I realized as I was sketching the SMC, that with out a reservoir to supply brake fluid, the SMC inlet would be on the receiving end of the high pressure brake fluid of the rear master cylinder, which could appear at the rear brake caliper.

Specifically high pressure brake fluid (from the rear master cylinder) at the SMC inlet would easily pass by the primary seal, increasing the pressure of the brake fluid in the remainder of the SMC cylinder, which in turn would reach the rear brake caliper, causing the two non-center brake pistons to activate. I'm referencing this sketch.

Is this correct? Perhaps I just don't fully appreciate the complexities of the Honda hydraulic brake system. Besides what's with the check valve in the SMC inlet, is that a pressure regulating device?

It's time for a field trip to the garage to conduct an empirical test of the SMC & the rear brake caliper.

• Motorcycle up on the center stand, engine off.
• Carefully prise the rear-most rear brake caliper piston back into the caliper.
• Insert a .125? shim between the afore mentioned caliper piston and the brake pad.
• Activate the rear master cylinder to extend the rear caliper center piston back into contact with the brake pad, until the brake pedal is firm.
• Remove the shim from rear brake caliper.
• Again activate the rear master cylinder, and observe.

Just as I had come to expect, the rear brake pedal required a couple of strokes before the pedal firmed up again. Once the pedal was firm, I checked on the position of the rear-most rear brake caliper piston, and it was now fully in contact with the brake pad.

Wow that was enlightening!

As a result of this experiment, I'm comfortable in saying:

1. There is no brake fluid pressure regulating device at the SMC.
2. Yes, brake fluid does pass by the primary seal of the SMC when the rear master cylinder is activated.
3. Surely if pressurized brake fluid can be moved past the primary seal of the SMC, then it stands to reason that the same pressurized brake fluid would activate the two non-center pistons of the rear brake caliper.

The check valve appears to allow greater brake fluid volume to pass into the SMC, yet only allow low volumes of brake fluid to pass back to the rear master cylinder hydraulic circuit from the SMC. Perhaps this aids in replenshing brake fluid to the rear caliper.

So back to the original question:

So does that mean that if all 3 pistons move as a result of operating the rear brake pedal we have a good indication that the SMC needs repair or replacement ?

No, I think it's intentional that the rear brake pedal activates all three pistons of the rear brake caliper.

However I do think that almost any play at the SMC, may be a bellwether of pending repair.

Sorry for the long-winded reply John.

 

[acedantinne]

If U activate SMC the center rear piston has pressure on it, out force. I want to show Dave something I have discover bleeding the ST1300 brakes. I read he,Ann's DaD, Dave, has looked into this SMC alot. My rear brake does not drag? have oem SMC. It is loose, so to speak. u can move it as in play up & down. No did not measure the distance.

 

[Anna'sDad]

But first a little check. I wanted to verify that the two circuits in the rear brak caliper are actually independant. I have a spare rear caliper that I bought from a breakers when I thought I would have to replace mine. Out came the pistons (very reluctantly) and I checked with pressured air. The centre bleed valve and the centre banjo bolt fitting are both connected to the centre piston. The outer banjo fitting and outer bleed valve are connected to the two outer pistons. There is no way fluid from one circuit can get to the other circuit inside the caliper. This was what I had always assumed.

Thank you for this observation. I also had 'assumed' that to be the case, now we know.

Then I start wondering about the use of the word 'seal'. It would be a bit of a misnomer if fluid under pressure is able to get past the seal (unless the seal is in need of replacement). In any case, these seals are designed to work under pressure. What happens to the fluid that manages to get past the seal - if this is what is happening. Presumably it would stay in the braking system causing brake drag as I think you pointed out before. That would be a problem.

Oops, I guess I didn't explain that well. When I stated

high pressure brake fluid (from the rear master cylinder) at the SMC inlet would easily pass by the primary seal

I was indicating that brake fluid under pressure could (should be expected) to slip past the primary seal as the pressurized brake fluid was on the 'back' (or non-sealing) side of the seal.

That of the slide valve inside a steam engine.

Interesting that you mention sleeve-valve engines, my grandmother owned a Mercedes Knight touring car, I remember playing in that automobile, though never saw it run. However I'm quite sure the ST1300 SMC does not incorporate such a system.

When viewing the sketch of the SMC, brake fluid under pressure (from the rear master cylinder) enters the SMC inlet, passing through the inlet port into the cylinder, note also that the same pressurized brake fluid is also passing into the cylinder (in front of the piston) via the compensating port, though at a much reduced volume. The pressurized brake fluid quickly creates a pressure differential surrounding the primary seal, with the greater pressure on the 'back' side of the primary seal. Due to the design of the primary seal, brake fluid easily slips around the primary seal allowing brake fluid pressure in the SMC cylinder to increase, causing the two non-center pistons of the rear brake caliper to activate.

I'm sure this is not a design flaw, rather an intentional and necessary feature of master cylinder design. A methodology to allow brake fluid to quickly pass through the master cylinder (primary or secondary) to supply the hydraulic brake circuit is needed. Imagine the time required to flow a litre of brake fluid through the tiny compensating port, when filling or bleeding the brake system!

What do you think ? I suppose a test would be to push all of the pistons in the rear caliper in a bit. Tie the front brake caliper so that the SMC is permanently activated, then operate the rear brake pedal. If the above is correct, the piston operated by the SMC should not move any further as the brake pedal is operated.

I think that pretty much regardless of the position of the SMC (at rest, or activated), anytime a pressure differential exists across the primary seal (where the greater pressure is on the 'back' side) brake fluid will pass by that seal increasing brake fluid pressure in the rear brake caliper circuit.

 

[Anna'sDad]

Got it. Apologies for the misunderstanding...

You're kidding right?

It's been months since I made that sketch, and only now that I think I've gotten it straight in my head.

 

[alienskydiver]

Well SMC replaced brakes all bled back brake still dragging pump the pedal the wheel will barely move after a few seconds you can just turn it I really am at a loss and am at my wits end . I need my bike for work using the car is driving me mad and taking me twice as long, I would be quite happy to push it onto the drive and burn it. I used to really like this bike but I am slowly losing interest I have spent the best part of ?200 pounds on it now and my budget is diminishing slowly

 

[Anna'sDad]

After carefully reviewing the many entries in this thread, I think I've managed to distill the pertinent data down to this:

1. Brake system was working fine until rear brake pads needed replacement.
2. After replacing rear brake pads, rear brake would not release fully, prohibiting rear wheel from spinning freely.
3. First effort:
   • Rebuild rear brake caliper with new seals, due to possible contamination during brake pad installation.
   • Bleeding air from rear brake caliper, yielded less than idea results:
   • The front bleed screw (single center piston circuit) when opened, released pressurized fluid, and air.
   • The rear bleed screw (two non-center pistons circuit) when opened, released nothing, no fluid, no air, nothing.
   • Pumping the brake pedal applies the brake pads, though when the pedal is released, the brake pads do not release. Opening the front bleed screw allows the brake pads to partially release.
4. Second effort:
   • Bleed the brake system in correct order with a vacuum pump:
   • Front brakes are fine.
   • Rear brake continues to drag when the pedal is released (though no longer locks up).
   • Noticed that the center piston appears to operate correctly (extending & retracting), however when the two non-center pistons extend to the brake pads, they do not retract.
5. Third effort:
   • Manually operated the SMC, it appears to operate correctly (extending & retracting).
6. Fourth effort:
   • Replaced the SMC, and bled the brake system. Pumping the brake pedal applies the brakes, releasing the brake pedal does not release the brakes preventing the wheel from turning, at a later time (seconds?) the wheel will turn, though with resistance.

I have a few thoughts on what may (or may not) be going on here:

1. Bleed the brakes until brake fluid is present at both bleed screws (rear brake caliper) and all the air has been purged. This will take more than one filling of the rear master cylinder (I recently ran a liter of brake fluid through the rear master cylinder trying to get the last bit of air to purge).
2. As the brake system was in working order before replacing the rear brake pads, it may well be that some action occurred during the brake pad replacement with unintentional results. I can think of two mechanical actions that are worth investigating:
   • Check that the brake pads are installed correctly, specifically that the brake pad tab at the front edge of each brake pad is properly located in the steel retainer (part no. 6, parts fiche). I have had difficulty in the past with fitting the inside brake pad into the retainer, apparently at some time I may have bent one of the tabs on the spring pad (part no. 5, same fiche), which greatly hampered installing the brake pads.
   • Check that the SMC bracket pivots (one at the base of the SMC bracket, and another where the SMC brake rod attaches to the front fork), are both clean of debris, and corrosion. Both pivots need to be well lubricated to ensure free and easy motion, not an easy accomplishment given their location, and minimal weather protection.
3. In the event that the problem is not mechanical, then it may well be hydraulic. As has been discussed (indeed highjacking this thread), if a master cylinder piston is prevented from retracting far enough to clear the compensating port of the master cylinder, then brake fluid cannot return to the master cylinder reservoir. Brake fluid (perhaps already pressurized by the master cylinder) unable to return to the reservoir will not allow the brake caliper pistons to retract. Indeed the brake fluid need not have been initially pressurized (brake pedal use) to cause a problem, brake fluid expands as it absorbs heat, and with no place to go, pressure builds, and the brake pistons extend causing the brakes to drag.
5. The Honda ST1300 rear brake circuit is unconventional in that it utilizes two master cylinders (the rear pedal master cylinder, and the secondary master cylinder). If either of these two master cylinders has a blocked compensating port, I can imagine the effect would be that of a rear brake that doesn't release.
7. Whilst diagnosing and resolving brake issues on this motorcycle can be frustrating (and driving the automobile is a drag), imagine how frustrating it will be every morning when you drive your automobile past the charred hulk that once was a motorcycle.

For the moment I suggest putting the matches and petrol away.
Take a look at the rear brake pads; with a bit of contortion, a strong lamp, and a mirror the inside brake pad and retainer can be seen.
Also see to it that the SMC pivots are lubricated.