Thanks for these two suggestions
@Oldbikefixr and
@Obo (as well as
@Igofar and
@Andrew Shadow before them).
Becasue it is an interesting problem, I'm going to keep adding my comments so that anyone reading int he future gets a full picture of hwo this was diagnosed as I (we) diagnose it. As yet we don't have the full answer, but I think I know what the full answer is. (At least I can guess, but I'm not footing the bill).
Now that is an excellent suggestion that I doubt that many people would think about immediately, and one which caught me out when I got my first ST1100 with the combined braking system. But since that one occasion, my first reaction to hearing that someone's back brake is sticking is always to head fro the front brake. Have they put rear pads in the front, have they put heat shields on the (new) front pads, have they put the rear retainer clip in the front caliper bracket. All of which will make the front brakes bind, activate a perfectly good SMC, and make the rear brakes very hot.
Another excellent suggestion, but, no I got him to check that early on. The symptoms that he described:
- The back brakes were dragging badly and after a short ride they came back hot. (Incidentally, the comment was that he went for a ride and only used the front brakes, but the rear was hot), which is a misunderstanding of how the ST1300 brakes work.
- The brakes do back off a bit in the garage if he forces the wheel to rotate. After that, he always had to pump up the rear brake with the pedal.
These two observations are at odds with each other - which suggested two problems to me. But it was important to identify whether or not the SMC was at fault. He couldn't get fluid to push back from the rear pistons to the bleed valve on the front center piston (which is directly connected to the SMC inlet port.) But he could get the fluid to emerge if he cracked open the banjo bolt on the SMC outlet port.
So that clearly identified the SMC. Which has been replaced now. The SMC is functioning. There is no fluid pressure to release when the back brake is binding. The SMC is doing its job properly relealsing the pressure back to the reservoir. (Which has plenty of air space).
So yes indeed the situation could be caused by a warped disc. With old pads and a properly moving caliper, you cannot feel a disc warp when braking. The caliper takes up the movement, and doesn't pass any feel to the pedal. Not so if you fit new pads. The caliper has no room to move with the disc warp, so the pistons pulse and you feel it under your foot.
But that movement when the brake isn't applied nudges the pistons into their bores, so every time you press the brake pedal , the first press is taken up by pumping the pistons back out again.
But he has checked the disk for warp and it is running true. I don't know how he tested it.
But that behaviour makes me think that when he applies the brakes the pistons are forcing the caliper to slide along pins which aren't parallel, so thaey are being sprung apart or together and keeping the caliperfrom returning. I also suspect that the pads may be not parallel to the disc surface for some reason - so that when the brakes are applied, the pads are forcing the caliper to rotate slightly - again jamming the sliding mechanism in place. We know that can happen if the pad retaining clip seat is worn. It results in the rifling appearance that Larry has described which he spent a long time testing and trying to diagnose. If I remember correctly, in that case the brakes were moaning as well. Since then, I know that Larry has become familiar with the signs of a damaged / distorted / worn caliper bracket and has replaced many since - all with excellent results.
One of the last things I found out during last nights call was that he is removing the slider pin to remove the calipers. I don't know why. And I don't know how he knows that the lower slider pin is in place. I had recommended that he didn't - but perhaps that is the way to get the caliper off without removing the pads ? I don't know. I always assemble bracket and caliper. Put the assembly over the disc/rotor and slide the axle through. Fit the pads last. Which is what I had previously described to him as 'the way to do it'. Leave the slider pins in place. Nothing good will come of removing them. So I don't know how he makes sure that the pads are located properly behind those tags on the pad spring. I need to find out about that before diagnosing duff bracket.
Interesting stuff.
