Rear brake is locking up.

[technician]

Quick update,
I've been back on the road for approx 3 weeks now after almost what felt like a complete rebuild of the braking system. Everything appears to be operating within normal parameters and behaving itself. I've even been away with the trailer on the back and no problems have arisen. I will not tempt fate and state everything is sorted but it's definitely a lot better than it was
Thanks again to all for the help and advice especially John Heath.

 

[jfheath]

You are most welcome, but it was you that got your hands dirty and had the courage to put your money where my mouth was !

Thanks for letting us all know the outcome, and well done for sticking at it.

 

[SMSW]

You are most welcome, but it was you that got your hands dirty and had the courage to put your money where my mouth was !

Thanks for letting us all know the outcome, and well done for sticking at it.

John, I assume from what @technician said, it was his SMC that was bad despite his rebuild?

 

[jfheath]

John, I assume from what @technician said, it was his SMC that was bad despite his rebuild?

Yes. It appears so.

When @technician first got in touch, he had suspected the rear master cylinder and had decided to replace that, and had ordered and fitted an SMC service kit at the same time, having cleaned out the compensation port. He had also already carried out a lot of remedial work - new pistons, seals, slider pins, pads, new rear brake disc/rotor.

Later conversations revealed that the original SMC piston had been stuck in the bore and had to be blasted out with air pressure. With the new service kit fitted in the SMC the piston appeared to have the full range of free movement.

But the rear master cylinder was a new entry to the system, so in my mind that was also suspect. I have been caught out one time too many by assuming wrongly that a new item had fixed a problem. I'm not one to launch into suspecting the SMC when other things can cause the identical symptoms.

If the fluid can't get back to the rear reservoir due to a rear master cylinder fault or a fitting problem, then it cannot get through the SMC either. So he went through a series of tests that I suggested and we found the culprit from those.

The clincher was the flow of fluid back from the rear outer pistons, which is worth repeating here.

• The rear outer pistons would not push in
• With no bleed valves open, the front left centre piston would push in relatively easily. Proof that the line from the SMC inlet to the rear master cylinder was not the problem.
• With the bleed valve at the PCV open, the rear outer pistons pushed in easily. This put the problem firmly in the line from the PCV up to and including the SMC, and cleared any suggestion of stuck pistons.

Fowlers, who I believe is the main importer of Honda parts in the Uk, had no SMCs in stock, but they got one to him within a week.

When you have had locked or overheating rear brakes, it can take a long time to get over the mental images of how badly this can turn out. In my case it has never gone away - and I still regularly check that I have no rear brake drag when a suitable stretch of downhill slope presents itself. With the engine not driving the bike, pull in the clutch. It should speed up. If not, what is holding it back ?

But yes. The service kit appeared to be the answer, and once again it seemed to be the full SMC that was required. Not to mention all of the other items in the braking system that @techician had already attended to.

 

[wls363]

SOLVED!!!
Hi folks, I posted in this thread a few weeks ago, working with Igofar to solve my rear brake dragging issue and overheating disk. After much testing, thinking and discussing, multiple dismantles, inspections and remounts of the brake assembly, I concluded my issue could only be mechanical (not hydraulic related) as the pads stayed squeezed when the fluid pressure was manually released. It made no difference if the braking pressure was applied using only the rear brake pedal and the 2 ends pistons, or if it came from the secondary master cylinder and the centre piston. That should have been the big red flag for me then and there. Sometimes you learn as you go, and you err.
I also replaced the new after market pads with the oh so expensive OEM's with no improvement... Every components of the rear brake assembly was apparently fine or at least not apparently defective, worn, bent... to my novice eyes at least.
I decided to buy a complete used caliper and support arm assembly on eBay from a bike salvage parts guy in Texas ( eBay ID cyclestripper ), as a new OEM set is very expensive, especially not knowing if I really needed it, and it takes 6 to 8 weeks to get one here in Ontario... The assembly (complete with arm and ready to install) looked clean and in good shape in the post pictures, and it was only 60$ + some shipping cost! Not much to lose at this point, right?

Well, good news! I am back on the saddle and riding again. Problem solved!

There is no way my support arm was bent or worn out (I had already replaced it just in case, for no gain). What could possibly go wrong with the caliper body? It is basically a big chunk or rugged metal... The pistons and seals were freshly replaced, cleaned polished and lubed. The only thing left I believed could have caused the jamming are the 2 guide pin bolts allowing the caliper to slide and maintain its proper position under load... Well guess what, the shop that did the caliper service at the beginning of this saga charged me (50$) for those 2 new bolts as part of the service kit, but they never actually replaced them, and did not keep them when I went back asking for them. Bad bad business!
I am glad this is over! This has been a challenging and frustrating job for my mechanical skill set.
Thanks to all of you that helped me understand how these linked brake work and what to look for.
W.

 

[jfheath]

Charging $50 for items they never replaced - and presumably never ordered ?

Glad it is all sorted though. Sounds like you are well up on the rear brake learning curve.

A couple of points for future reference:

It made no difference if the braking pressure was applied using only the rear brake pedal and the 2 ends pistons, or if it came from the secondary master cylinder and the centre piston.

Probably a brain fart, but you have written this the wrong way round. The secondary master cylinder applies pressure to the 2 rear outer pistons, not to the rear centre piston.
The rear pedal applies pressure to the centre piston via fluid in the line that runs direct to the rear caliper.
When the bike isn't moving, the rear pedal also applies pressure to the two rear outer pistons as the fluid in the second line to the front calipers is able to pass the primary seal in the SMC.

I concluded my issue could only be mechanical (not hydraulic related) as the pads stayed squeezed when the fluid pressure was manually released.

Good diagnosis for your problem.

However, again for future reference, the same symptom is often a sign that the SMC piston is stuck in its bore - often due to corrosion caused by water in the brake fluid as a result of infrequent and incorrect servicing. When this is stuck it prevents the pressure from releasing the two rear outer pistons. So although the problem is mechanical at the front, the pressure on the rear pads in this situation would be due to hydraulic pressure.

You have done well to diagnose that problem. Its all too easy to blame the SMC for rear wheel lockups, when other mechanical issues - as in your case - are equally likely. The SMC test is easy - if you apply it by hand and the rear wheel locks and releases, then it is doing its job, so start looking elsewhere.

Of course the real issue is that the brakes are getting old and worn, and may be suffering from poor quality servicing so all options are open.

I hope you don't mind me correcting your post, but the more accurate information people have, the less they will have to rely on scumbags who charge $50 for parts that they haven't fitted.

Do you have any idea as to what caused the issue with the guide pins ?

 

[mortenk]

I once removed one guide pin for proper cleaning, and installed it slightly cross threaded. It wasn't enough to be obvious, and it went in without too much trouble. That caused major issues for my rear brakes for the next few weeks. The pads would wear unevenly, and they would drag due to the slightly wrong angle. My pads wore out quickly and very unevenly (one end worn, the other not, on both pads). It was actually the wear on the pads that made me look at the guide pin an extra time, and after removing it and installing it carefully ensuring it went in correctly, the problem was solved. I was really surprised how easy it was to install it incorrectly. Maybe the dealer did the same to yours? Glad you got your problem solved!