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pressure loss on rear brake

Agemax

Agemax

Forum Guru
my rear brake gradually loses pedal pressure over a period of time. there are definately no leaks so am i right in saying it is the slave cylinder to blame rather than the caliper?

cheers
 
what bike ? On my 77 and 78 i used the load it up and let it set method on both the front and rear masters. for the front one you simply pump up the brakes as hard as you can get and hold the lever back to the handle as tightly as you can and wrap a small bungie cord or a zip tie around he lever and grip. Leave it sit over night in this overly depressed state. On the back one i did the same and wedged a screwdriver handle between the clutch case and pedal to keep it depressed and loaded up.. This procedure causes the rubber cups to be pushed very tightly against the cylinder walls.. let it set overnight and youll be amazed at how much better your brakes are.
 
Last edited:
Rear Brake

Rear Brake

Brake fluid is getting by the piston seals in the cylinder. Get a kit and install it after thoroughly after flushing the system out. Wipe the inside of the cylinder and the reservoir out well. It is most likely that there are bits of seal material in there somewhere. There are plenty of kits out there and replacing these parts is a snap.
 
Before dumping money on a rebuild kit, pull out the piston and look at the bore for pitting. If it's pitted, it's junk - rebuild kit is a waste of money.
 
Pitting

Pitting

Pitting is definitely a death sentence for the cylinder. I once had to replace one on a '77 750 because of scoring, which was kind of weird. I couldn't find anything solid enough that would have done it. Maybe it dissolved eventually?
 
Could also be a leak on the master cylinder. Rub your finger around the shaft that runs from the pedal up to the MC - if you can feel fluid on there it's leaking.

If it's the caliper and it needs new pistons get the stainless ones from Wemoto. Problem solved for ever.
 
Why would the caliper be junk because of pitting since the piston should only come into contact with the seal? We just trashed two pistons because of pitting (pitting will tear the seal). Pitting was in areas that would be wiped by the seals. As long as the pitting on the caliper isn't in the cut where the seal goes and the pitting isn't pervasive (would cause the piston to cant to one side or the other, we are talking extreme) or cause a leak, don't see the harm. Anybody ever have their calipers sleeved like the corvette guys?
 
Pitting

Pitting

Calipers are more apt to be pitted because of their exposure to the elements. The tucked away cylinder getting pitted is not as common, with the rubber cups usually being the problem.
 
hampshirehog said:
Could also be a leak on the master cylinder. Rub your finger around the shaft that runs from the pedal up to the MC - if you can feel fluid on there it's leaking.

If it's the caliper and it needs new pistons get the stainless ones from Wemoto. Problem solved for ever.
Click to expand...
there is no evidence of a leak either on the caliper or the master cylinder.
the caliper has recently had new seals and pistons so im going to get a rebuild kit for the GSXR master cylinder
 
BILLDIGIOIA said:
Why would the caliper be junk because of pitting since the piston should only come into contact with the seal? We just trashed two pistons because of pitting (pitting will tear the seal). Pitting was in areas that would be wiped by the seals. As long as the pitting on the caliper isn't in the cut where the seal goes and the pitting isn't pervasive (would cause the piston to cant to one side or the other, we are talking extreme) or cause a leak, don't see the harm. Anybody ever have their calipers sleeved like the corvette guys?
Click to expand...

That seems logical to me to.
 
Agemax said:
my rear brake gradually loses pedal pressure over a period of time. there are definately no leaks so am i right in saying it is the slave cylinder to blame rather than the caliper?

cheers
Click to expand...

"gradually loses pedal pressure over a period of time"... are we talking 'time' during which pressure is maintained on the pedal or a matter of hours or days between use?
 
Agemax said:
after a couple of days with no use
Click to expand...

Oh. If you have replaced the caliper seals (rotating seals) I don't know what else would cause that.

If the pistons were (for some reason) moving away from the disc, any slack created should be taken up by the first squeeze. Hopefully the MC re-kit will solve it. Good luck
 
Colin Green said:
Oh. If you have replaced the caliper seals (rotating seals) I don't know what else would cause that.

If the pistons were (for some reason) moving away from the disc, any slack created should be taken up by the first squeeze. Hopefully the MC re-kit will solve it. Good luck
Click to expand...
cheers Colin. bought another master cylinder and a new seal kit off ebay. that should cure it hopefully
 
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