Hi All
Go easy on me, first time on one of these forums.
Apologies if this has been covered elsewhere, but if the specifics I will mention have, then after hours searching I've not found it! My explanation will probably be long, but I'm hoping that in response all I'll need is a quick 'yup, you're right!' and that my description and any responses will also help others.
I am in the process of restoring a 1976 CB500T...the front brake is I believe the same as those on the 1970s SOHC 4s. No matter what I do the lever goes back to the bars and although the front brake operates and releases again, there is not quite sufficient stopping force.
I am pretty certain that I need to rebuild, or perhaps better replace, the MC but before splashing out any hard earned dosh wanted to see if there was agreement that I had got it right.
So in summary, there are only a limited number of components that could be faulty, the caliper, brake hoses, and MC, plus the brake fluid.
I've done numerous tests to try and determine which part is at fault, and on the face of it, individually each is okay, they just don't work together...weird...but hopefully I've fathomed it out and my conclusion at the end is good?
Okay, so I'd broken the bleed nipple off the original caliper. Local company that specialise in vintage vehicle brakes removed the old nipple and replaced it, plus fitted a new seal and piston.....so the caliper is probably now better than new.
The rubber brake hoses looked fine, no sign of ballooning, and there was no leaks at any of the joints, or from the MC or refurbed caliper.
Refilled with fresh dot 4 brake fluid, bled the air, and tried the brakes, which I cannot get any better than I mentioned at the start. So I started trying various tests.
Firstly, no amount of pumping on the lever improves the braking, which I believe confirms there is no air in the system, as if there was I could have temporarily pumped the brakes up. I tried everything bleeding anyway, including using gravity, then clamping the lever back partially overnight to allow air to escape.
If I clamp the bottom hose just before it enters the the caliper, and try to pull the lever it is rock solid. I believe this confirms that the hoses and joints throughout the system must be fine (and also no air between MC and clamp).
I also believe that this test is supposed to confirm that the seals in the MC are good.
So at this point, all the components appear to be good, yet put together the brake is rubbish.
Next, I wondered if the connector block between top and bottom hoses was the problem. So I removed the master cylinder, mounted it on a piece of pipe in lieu of the handlebar, and connected the bottom hose directly into the MC, and bled the system.
The feel of the brake like that is exactly the same as when the whole system is connected, which I believe confirms that there is not a problem with the top hose or connector block/brake light switch.
If I clamp the hose, anywhere but not quite so tight that it is completely blocked off but tight enough that it effectively creates a non return valve, I can get enough pressure into the system after a second pull on the lever to get sufficient stopping power into the brakes.
Just to clarify on this test, the clamp is only fractionally less tight than with the hose fully 'squeezed'. With a good strong pull on the lever fluid can be forced past the clamped part of the hose, but there is not sufficient return force from the caliper to force fluid back past the clamped bit.
Looking at the caliper when I do this, on the first pull of the lever the piston/pad moves out of the caliper the same amount that it would normally without the clamp, but does not then return when the lever is released. On a second pull of the lever, which will now only travel a relatively small amount (nowhere near to the bars), the caliper piston/pad moves out a shade more and grips the rotor like it should.
I believe this test confirms that the caliper is working okay, but could suggest that the hoses are expanding, or seals inside the MC are shot. However, the integrity of the hoses and MC seals has already been confirmed by the lever becoming solid on full clamping.
So at this point, the various tests suggest nothing wrong with any of the components, yet the brake is till absolute rubbish.
However, somewhere on another thread on here I think I found the answer to this mystery. It was suggested that some MC repair kits come supplied with pistons that are an incorrect length, and whether too long or short can cause problems.
If too short, there is loads of travel before the relief hole is covered, so only the final part of the piston movement generates any real pressure in the system.
If too long, there will not be enough movement before it hits the stops to generate sufficient pressure in the system, plus as the relief hole might permanently covered, once applied the brakes will not be able to release, same as if that hole were blocked with crud.
Both possibilities would give similar problems with lack of system pressure and thus braking power.
My caliper releases just fine, so I am starting to think that a previous owner has rebuilt the MC using a kit where the piston is too short. The last owner has confirmed that the front brake was always well below par, and that it must have been someone before him that rebuilt them. if indeed that was done.
Also, when I take the reservoir cap off, fluid squirts quite violently back up from the relief hole throughout most of the pull on the lever, which I think again suggests that the piston is too short.
So my conclusion is that the MC has probably been rebuilt at some stage with a piston that is too short. Even if that theory is wrong, the various tests tend to confirm that there is nothing wrong with the caliper, hoses, and that there is now air in the system, so the only thing left to repair/replace is the MC.
If there is agreement that the problem appears to lay with the MC, I think I'll just get the pattern replacement from David Silver as it is only marginally more expensive than the repair kits but avoids the hassle of getting that flipping circlip out, the chances of messing up, or being supplied with the wrong piston.
Does it sound like the MC is at fault and if so due to an incorrect piston or some other reason, but if not, has anyone any other suggestions for what is wrong?
If you have got to the end of this essay, well done, and thank you so much and you deserve a long cool beer....cheers.
Thanks again for your help
Yaz
