Hey All,

I'm a newbie from the UK.

I had a couple of questions you guys might be able to help me with. I have a `90 GS. The front brake is poo... I've bled it, had a new caliper put on and got new brake pads but the lever is still coming back to the bar without much effort and the brakes seem to have more to give...

A friend suggested that the master cylinder might well be the problem. What do you reckon? Is it worth stripping and reassembling or better to get another one?

Also has anyone upgraded their master cylinder with a bigger diameter one?

Hope you can help,

Cheers,

Adam  

New caliper or new USED caliper? Sounds like you don't have enough pressure in the system, most likely due to either air in the system or a leak. A larger master cylinder reservoir won't help if air is getting in somewhere else. Are you seeing fluid anywhere? Check all the hose fittings and the o-rings in the caliper (I've never needed to mess with my GS caliper, but the o-rings in my race car brakes need to be replaced every year and the symptoms of that are what you describe. The let just enough air through to make the brakes mushy.)

Try getting some Steel Braided brakelines might be that your old ones are expanding when you use the brake, also sounds like you have air in there.

It was a new used caliper which seemed in good condition - tried out a couple down at a recent bike breakers and this seemed the best with all the seals etc intact.

There do not seem to be any leaks at all with no sign of fluid... I already have steel braided brake lines. The whole system was bleed extensively by a guy at the breakers who new what he was doing. It was then that it was suggested that the master cylinder was not working quite right.

By a larger master cylinder i meant with a large diameter bore, rather than larger resivoir... or have I misunderstood??  

Cheers for the comments so far  

Adam

I suppose the master cylinder could be the culprit, but the stocker actually works fine if it's operating properly, so I don't think a larger or different cylinder is the answer. You're losing pressure somewhere in the system.

The master cylinder piston is probably the problem.  Either the piston isn't sealing in its bore or the the little one way valve between the piston are resivour isn't doing its job.
You might be able to find a MC rebuild kit, but its usually easier to just buy a good new or used one.  

I would advise against changing the bore size on the one you get though. I'm not a hydralics expert, but I've found it doesn't work all that well.  A smaller bore is easier to pump but doesn't exert enough pressure and bigger is harder, but doesn't generate as much pressure. If that makes sense...

You're SURE that the mech bled the entire system without ever emptying the master cylinder?

I've heard that if the master cylinder ever goes dry while you're bleeding down by the caliper, you may need to get one of those MityVac (sp?) units to help you get around the "vapor lock" up near the master cylinder.

Does this sound familiar to anyone else?

Have ordered a second hand master cylinder in good cond, so will have to see if that sorts it... ANy tips for fitting a new master cylinder???

Cheers for all the advice,

Adam

Hey Guys,

Just wanted to let you know that the new (but used) master cyclinder seemed to do the job great! Front brake feels much better now - can't wait for the progressive fork springs next week to make it even better at stopping!

Cheers for all the help,

Adam

Quote from: KerryYou're SURE that the mech bled the entire system without ever emptying the master cylinder?

I've heard that if the master cylinder ever goes dry while you're bleeding down by the caliper, you may need to get one of those MityVac (sp?) units to help you get around the "vapor lock" up near the master cylinder.

Does this sound familiar to anyone else?

Kerry,  I had and have the same problem.  I rebuilt a bike that sat for years.  I rebuilt the front caliper and decieded to flush out the brake system and replace with dot5.  After that it was mushy as adstar has described.  I had a hard time getting the fluid to start flowing.  It finaly flowed and I bled that heck out of it and it was still mushy.  I then rebuilt my master cylinder and re-bled but still mushy.

I just bought a vacumm pump from Auto Zone, I will try that next.  If that doesn't work, I have a complete front system from a 94'

It does sound like air in the lines or trapped up in the master cyclinder.

Paul

If the master cylinder is worn it won't build up enough pressure.
It doesn't get air in the system.
It just allows fluid to get past the piston back into the reservoir, instead of forcing to the caliper.
The clean brake fluid could have washed off some buildup that was actually sealing the worn part.

You can mount a gsxr master cylinder as a replacement. You use the GS clamp piece instead of the gsxr part. That way you still have a mirror mount. A piece of aluminum flat bar bent and drilled will mount the remote reservoir.
Works great.

If fluid is leaking past the piston in the master, the handle should slowly go to the grip, right? I my case, the handle will go only so far (mushy) but stops at the same place.  If I hold it for a while it still doesn't go any futher.  This is a GOOD sign... Isn't it?

Paul

The piston in the mastercylinder has a certain amount of travel.
It will bottom out eventually.
If your brakes work and the lever stops at the same point each time, then that sounds normal to me.
You can put a gsxr lever on the GS mastercylinder. These have the adjustment knob to move the lever out a little. If your lever comes too close to the bars, this would move it out farther.

Thanks Bob!  I think it's air in the system.  I'll bleed it with the power bleeder.

And, thanks Adstar - for letting me whore-it-up on your thread.

Paul

QuoteAnd, thanks Adstar - for letting me whore-it-up on your thread.

Paul

No worries Paul - glad it helped someone else.

Adam