A couple of additional snippets.
Reversing brake groan. Mine does that nearly every time I take it out of the shed. It seems to be caused by light surface rust on the disc surface - it is bare metal, free from any grease - it is going to rust - initially, a light coating which disappears with the first application of the brakes. Question should be, does it do it when you put it away ?
Putting silicone grease behind the rubber boot. Make absolutely certain that the boot is replaced correctly so that it seals. With the SMC all coupled up, I am unable to do it. I have only ever been able to do it with a very firm push from an appropriately sized cylindrical tool, and that means taking off the bracket that you should never remove. I’ll come back with a link.
https://www.st-owners.com/forums/th...e-smc-boot-fitting-a-cause-of-failure.174587/
One reason that the SMC causes rear brake lock up, is the the SMC piston cannot return to its correct position. The tiny compensation port is not cleared because of this, and fluid cannot escape. So the rear caliper outer pistons remain out - pressing the pads against the rear disc. Normally when you press the brake pedal with the bike stationary in the garage, both brake lines from the rear master cylinder are in use. The direct line to the middle centre piston and the long line to the front calipers,
through the SMC and back to the rear outer pistons. So the rear pedal operates all three rear caliper pistons. ‘Through the SMC’ is possible because the primary seal of all master cylinders only seal in one direction - ie when pumping fluid. On the return stroke, they yield to let fluid from behind pass - rather like the leather seal or rubber seal in a bicycle pump. On the upstroke, it lets air past the seal ready for the next downstroke.
But you cannot test your SMC operation with the rear brake pedal. The SMC piston should return by itself - pushed back by the spring. In the case of corrosion or gunge build up, it will not be able to do this and the rear brake will lock on. However, if you press your brake pedal, the fluid pressure cannot move the piston forwards, but the build up of pressure can move it backwards more forcefully than the spring can manage - forcing it past any debris, and clearing the compensation port.
The test with the rear wheel and applying the SMC by hand must be done without involving the rear pedal. And there are three tests: Does the wheel spin ok; Does the wheel stop when I apply the SMC by hand; Does the rear wheel release instantly when I release the SMC.
If you have been using the rear brake then do the above sequence a few times.
That snippet of information may give a clue to a developing SMC issue. If the first time you apply your brakes with just the pedal, does the pedal dip further than before ?
If you release the pedal and apply it again does it feel ‘tighter’ this time. Ie you don’t have to press it as far.
If that is the case it may suggest that the SMC piston isn’t returning fully after it has been applied. The dip in the pedal is caused by the first press moving the SMC back.
But the same behaviour can be caused by a slightly warped rear brake disc.
