Advice needed on what is causing rear pedal pressure lose.

[Langley863]

So I need so advice on what could be the issue.
I recently rebuilt my SMC and bled the brakes in sequence and tilting the front left caliper.
Now the front brake works perfectly and the rear works perfectly however it I apply both and then release the front brake such as coming to a stop and keeping the rear down I lose all pressure and have to pump the rear brake once and then it's perfect again, no biding.
Now on centre stand I applied both brakes wheels stop I push on the smc the rear stops and then free wheels once smc is released but I can't seem to figure out why when I'm riding the rear loses pressure.
I have zero brake binding and all works as should.
I also have a clunk sound coming from the smc at slow speed but dose disappear if I apply front brake. Might clunk at higher speeds but I can't hear it.
My thinking is the smc bore has ovalled and changing the seals wasn't the issue and the piston is still sticking. My other thought is air in the system still but I just haven't got it out after 3 bleeds.

Any advice would be appreciated.

Cheers jono

 

[Gus1300]

@Igofar or @jfheath will be along soon...

 

[dduelin]

Pedal “drop” that occurs when you trail off front brake while still applying rear brake is caused by air in the circuit supplying the center rear piston. A good and complete bleed of the system from A to Z will remove it.

 

[Sadlsor]

I'm thinking improper bleed, as well.
The sequence is detailed and specific, and shortcuts will not allow for success.

 

[Igofar]

It could still be air in the PCV, but my question is, you said you rebuilt the SMC? Does that mean you replaced the piston and seals, and possibly cleaned the bore? Did you happen to check the screen cartridge on the back side of the old SMC?
This filter and the small return port often gets clogged, and while it may still stop and release the rear wheel when tested, it may obstruct the flow causing issues as described in your post.
Most rebuilds often require complete replacements to fix these issues.

 

[Slydynbye]

It could still be air in the PCV, but my question is, you said you rebuilt the SMC? Does that mean you replaced the piston and seals, and possibly cleaned the bore? Did you happen to check the screen cartridge on the back side of the old SMC?
This filter and the small return port often gets clogged, and while it may still stop and release the rear wheel when tested, it may obstruct the flow causing issues as described in your post.
Most rebuilds often require complete replacements to fix these issues.

Is there a bleed valve on the proprtional control unit?

 

[Igofar]

Is there a bleed vave on the proprtional control unit?

YES, number three bleeder in the correct sequence.

 

[MidLife]

I recently rebuilt my SMC .....

My thinking is .....

Well observed for a first time poster!

A bit of history on the bike and maintenance and SMC and environment might help also.

And how did a 32 yo end up on a ST? Did he loose a bet?

 

[Slydynbye]

I always wondered if the 1100 would benifit from one off of a 1300

 

[k2ramair]

I have read everything I can find about bleeding brakes on this forum. There is tons of great info. The ST brakes are unusual, but work excellent when when working proper. I read countless times that rebuilding the SMC rarely yields the desired results and it should just be replaced. Your rebuild may be good, but you might acquire a spare just in case. I have a spare, not because I rebuilt mine. Let's just say I have not bled the brakes as often as I should.

Here are some links that may give you some ideas:
ST1300 Brake System Simulator.
ST1300 Maintenance Getting All the Air Out of the Brake System
ST1300 Maintenance Brakes Avoiding the Pitfalls
Read any IGOFAR wisdom

Check valve bleed tool seems to be a favorite. Vacuum bleeders are a mixed bag. I like it because it is easy to suck the fluid out of the rubber hose when I pull it off the bleed nipple. My favorite is holding pressure on the brakes and opening the bleed nipple. When I feel the pedal or handle relax, I close the bleed valve. You end up reaching around to different part of the bike doing this unless you have a helper. For the SMC, I had the best luck with the following (must be removed and tilted appropriately):

1. Hold pressure on the SMC
2. Open the PVC bleeder valve
3. Close the PVC bleeder valve
4. Keep pressure on the SMC and refill the SMC by pumping the rear brake - it will extend the SMC
5. Repeat until no bubbles and everything is smooth - Keep an eye on rear brake fluid level

Have plenty of brake fluid and make multiple laps until you consistently see no bubbles through the 7 step process. I feel like you can forget about the front brakes when they are good and focus on the rear/SMC. However, I have never seen it mentioned it is OK to start skipping the Front hand lever process (Nipple 1 and 2). Perhaps lap the 5 bleeders for the rear/SMC brake until they are good, then go 1 through 7 to ensure everything is solid.

I am super thankful for all the work people on this forum have contributed to help me keep my favorite bike rolling after almost 20 years (17 in my hands).

 

[Langley863]

It could still be air in the PCV, but my question is, you said you rebuilt the SMC? Does that mean you replaced the piston and seals, and possibly cleaned the bore? Did you happen to check the screen cartridge on the back side of the old SMC?
This filter and the small return port often gets clogged, and while it may still stop and release the rear wheel when tested, it may obstruct the flow causing issues as described in your post.
Most rebuilds often require complete replacements to fix these issues.

Yes it was the piston kit part number 45620mcsg04 I didn't rebuild it myself however as I bought refurbished. I do however have my old smc which worked correctly but only swapped due to the inside seal beginning to leak fluid.I did check it however and all the parts where new including the rubber boot and c clip.
I will try and bleed the brakes again.

 

[Langley863]

Well observed for a first time poster!

A bit of history on the bike and maintenance and SMC and environment might help also.

And how did a 32 yo end up on a ST? Did he loose a bet?

Sorry history wise 2002 non abs 61000 miles.
I go touring and to lots of rallys and then the misses decided she wanted to come so needed the size and luggage but currently feel like I've lost a bet with this issue

 

[Andrew Shadow]

but currently feel like I've lost a bet with this issue

It is not a big issue regardless of what the cause is.

First you need to clarify your description of what is happening so we are certain of what you are talking about.
If you mean that when it is under pressure the rear brake pedal drops when you release the front brake lever but there is still rear brake function, that indicates that there is still air in the system as Dave described above.

If you actually do mean a complete loss of rear brake pressure and function as you wrote, that is a different matter. There would have to be a huge amount of air in the system to cause that, which is unlikely if you bled it properly as you state that you have. That symptom is more indicative of a defective rear master cylinder but again, that does not match the symptoms that you described.

Overhauling the SMC has been proven to be ineffective much more often than not. It is not recommended by most on this forum. A defective SMC can also cause all kinds of strange symptoms that you would not expect. If this started with an SMC issue, I would replace the SMC assembly with a new one from Honda, then perform a complete flush and bleed of the entire brake system following the instructions to the letter. Once that has been accomplished, I would re-evaluate and see how the system is responding.

 

[MidLife]

Yes it was the piston kit part number 45620mcsg04 I didn't rebuild it myself however as I bought refurbished. I do however have my old smc which worked correctly but only swapped due to the inside seal beginning to leak fluid.I did check it however and all the parts where new including the rubber boot and c clip.
I will try and bleed the brakes again.

Where did you purchase the rebuilt SMC from? Even if it is from a source with a good track record, it would still be worthwhile checking it like our guru Igofar advised earlier (many of us here listen to what he has to say), before spending more time bleeding and rebleeding and time away from your Missus.

 

[MidLife]

If he goes for a re-bleed, he could leave the front (outer pistons) out of the procedure, right? As long as he is happy with how firm his brake lever is.

No need to touch the front outers if he is chasing an SMC issue?

 

[MidLife]

Maybe there is somebody close enough on his island he could ride up to for first time guidance on a good bleed?

 

[Igofar]

If he goes for a re-bleed, he could leave the front (outer pistons) out of the procedure, right? As long as he is happy with how firm his brake lever is.

No need to touch the front outers if he is chasing an SMC issue?

Nope, you'll have to do all 7 bleeders in the correct sequence to get all the air out, or you'll just be pushing it around different paths.

 

[Langley863]

I've been outside most of today bleeding it and I believe the issue was air. Hopefully got it sorted going to test later but the piston in the smc has become a lot more reactive when pumping. I've also borrowed a vacuum bleed kit off a mate so hopefully that's helped. Thanks for all the advice so far I'll keep everything updated to if it works

 

[MidLife]

Nope, you'll have to do all 7 bleeders in the correct sequence to get all the air out, or you'll just be pushing it around different paths.

I would need help understanding that.

Isn't the front circuit completely isolated from the rest of the ST1300 braking combobulation?

If there is no communication between the front circuit and the rest of the labyrinth, it is not immediately obvious (to me!) how air could migrate from one to the other?

Would have thought that after working on the front MC for instance, all would have been fine with only a step 1 + 2 bleed afterwards.

Or conversely, after any work on the SMC loop, a bleed starting at step 3 could have been sufficient?

 

[Larry Fine]

This looks like a good opportunity for someone to find and post the braking system animation.

Is this it?

Or this?