Hi All, Time to consult the collective wisdom of the forum once again. I have a pair of 4-pad calipers and a 999 brake master cylinder fitted to my ST4s. I'm having trouble getting a good solid brake lever. I've just rebuilt the callipers with new pistons, seals and brake pads, and so have changed the fluid. There's no air in the system but with a good squeeze I can pretty well get the lever back to the bars - which doesn't inspire confidence. It was doing this before the brake work, which I undertook as the pads were worn, and upon inspection the pistons were starting to corrode. Can someone could advise how much travel they can get out of a good, working 999 master cylinder setup?
Has your brake set up ever worked properly ? The 4 pad caliper was fitted to the 748R and I believe at some stage the 996. If the master cylinders for those bikes were different to the 999 master cylinder, that could be your issue. There is also a very real possibility you have air trapped in the master cylinder which without a bleed nipple is a sod to disperse. Andy
It's never been great, which is sort of why I'm asking. As far as I know, the whole system is from an early 999, so it has the radial master cylinders with bleed nipples, they originally had the coffin-type reservoirs. I bled the calipers first, then finished off at the master. I have repeated this a few times so am pretty happy that there isn't any air in there. Just in case, I have tied the lever back to the bars and will see how it is tomorrow. The calipers have the 34mm pistons, and from a rough measurement the master cylinder is an 18mm bore? Does that sound right?
I run the same 4 pad caliper with an 18x20 radial master cylinder having originally used a 19x20 which had almost no movement at the lever. You need to bleed the master cylinder first though. Andy
OK, thanks for that, Andy! I'll try bleeding again in the correct order. With your 18x20 how much lever travel do you get? When these cylinders wear out, do they go spongy or do they generally just leak?
It's a 2 finger pull and I have adjusted the span so I'm about an inch from the throttle on full pull. My only experience of this was my clutch m/c which failed. It went spongy and under constant load went back to the handle bar. No amount of pumping would bring it back. Andy
It really sounds like air in the system if the lever is coming back to the bar, have you tried reverse bleeding the system? I think you will see an improvement overnight after tying back the lever.
I was going to suggest tying the lever back,which you are doing.Hopefully this will help.It did with mine.
So to follow up on this, I untied the lever this afternoon, and the brake feel was slightly better. I re-bled the system starting with the master cylinder, and then at the calipers. No real change. I didn't manage to reverse-bleed the system as I couldn't find the syringe I thought I had. Then I noticed that the span adjuster on the brake lever was seized at it's closest setting. With that unseized, I can at least get the lever to stop before it touches the grip. Total movement of the at the ball-end of the lever is just under 60mm, which is still more than I'd expect. I'll keep an eye on it and see how it goes.
