Brake master cylinder leaking after pad change?

I've just changed my front pads, and noticed there was a load of brake fluid that had accumilated in the right side hand guard. On closer inspection I could see the fluid had leaked from around the bottom of the dust boot on the master cylinder.

when I fitted the pads I'd pushed the pistons back a fair way to make room for the thicker pads, could this have caused the master cylinder to leak? I would have thought that there would be greater hydraulic pressure when braking hard due to the leaverage the brake lever gives than me pushing in the pistons manually.

Has anyone else experienced leaking when changing the pads? It is difficult to tell if it is still leaking as there is an accumilation of fluid trapped behind the rubber boot, but if it is knackered do BMW do a repair kit, or do I need to fork out for a new master cylinder?

Any advice gratefully received

Cheers
Dan

ps bike is '98 1100GS

I've not done it myself, but yes, new pads will push fluid back into the master cylinder and I suppose with enough force will leak out. The four screws holding the lid down aren't that tight.

I believe you're supposed to remove some fluid if it's quite high when you put new pads in. You should be able to clean everything up, dry the rubber boot and lid thoroughly, get the fluid level correct then reassemble.

HTH

Adam

Unfortunately it didn't leak past the lid, it was leaking from the boot around the piston, that the brake lever pushes in when pressed.

Aah, bugger. Just how much force did you use to get the new pads in?

Someone with a little more technical know-how than me will need to take over now. Hope it isn't too bad.

Adam

adamski49 said:

Aah, bugger. Just how much force did you use to get the new pads in?

Click to expand...

Not that much force! They moved back fairly easily, maybe it was a coincidence and the master cylinder was already leaking. I still think there probably would be more fluid pressure when braking hard than pushing the pistons in to change the pads.

I think that someone has topped the master cylinder up between pad changes. Thus, when you pushed the pistons back for the pad-change, the fluid will have had to find a way out past the master-cylinder seals. This can happen because the seals are 'lipped' - i.e. will only seal properly in one direction. Remove excess fluid and monitor the level carefully over the next couple of weeks.

Very common problem in 850 and 1100's. Piston wears very quickly for some reason.
Very easy to sort. If the master cylinder bore is ok, you just need a new piston and possibly a new boot.

Do not remove master cylinder.
Remove hand guard and lever.
Undo grub screw that retains piston and wiggle piston out.
Clear gunge that caused piston wear from piston bore.
Replace all above.
Lever/piston adjustment screw adjusted by tightening to remove all play, then tighten a further half turn, with locking compound on thread.

I have had to do this on by 1100gs and my 850r. In both cases, did not have to drain system down. When you have finished, open up caliper pads to force fluid back up into master cylinder, top up, wiggle lever to relaese any small bubbles.

Kit avaiable from motorworks, about £35. Apparently one of their best sellers!

Thanks for the advice

I'll monitor for a week or so, then replace piston if still leaking.

Great idea from Sea Lion opening the calipers to get the air out, saves messing about bleeding the brakes.

Cheers