Rear brake binding

[Mouse]

My girlfriend's F650GS has developed a "sticky" rear brake. After any distance at motorway speeds, the rear caliper is too hot to touch.

Usually, this would be caused by a dirty piston in the caliper, so I stripped it down and cleaned it meticulously, cleaned the seals and their grooves, and applied a smear of grease. Not that it was dirty, in fact it was the cleanest brake caliper I have ever stripped.

So, I investigated the master cylinder. The piston seemed to be sticking part way when the pedal was released, I assume this was blocking the return path for the brake fluid, hence leaving pressure in the circuit. So I stripped the master cylinder. Gave the piston a very gentle going over with 1000 grade emery, lightly applied brake grease, and reassambled. All seemed to be OK, the piston looked like it was returning properly.

But Clare just phoned me to say the caliper is hot again. So I'm stumped. I suppose it's possible the master cylinder piston has tightened up again - it runs inside a nylon (?) bore, perhaps this has swelled for some reason? If so, can it be replaced?

Can anything else cause these symptoms? I can't see what (it's a non ABS bike).

Cheers for any advice.

 

[Mzokk]

Letting her boot sit on the brake when riding along? Just a thought but I suppose you'd have noticed the brake light on??

 

[Mouse]

Already eliminated that possibility, but thanks

 

[Twotter]

How is the disc itself - maybe it's slightly warped and only becomes really noticeable after it has warmed up a bit ???

 

[Uncle Rob]

Mouse have you checked that the brake pipe is not holding the caliper tight
ie there is some slack in the pipe

 

[Mouse]

I don't think it's the brake pipe - the 650 has very flexible rubber hoses not the braided hoses that my 1200 has. And there's plenty of slack in the caliper end of the hose. Still, it's something I'll check again

Have a nasty feeling this problem is going to need a new master cylinder.

 

[Spout]

I needed a new master cylinder on my 650. Seems to be a commonish problem (see the Chaingang site).
I think I've still got my old one if you want it to repair then have a spare.

 

[Mouse]

Did you need a new master cylinder for the same reason (piston sticking)? Cheers for the info, I'll check the other site.