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difference between brake a clutch masters?

PostPosted: Fri Jul 15, 2011 5:31 pm
by mini64
I always thought brake master cylinders had the secondary seal and piston that acted as a valve to minimize back flow when the pedal was lifted off, so the pads would stay close to discs as they wore. Whereas clutch masters have the one simple seal and piston. Is this wrong? If not, then why is it when you look at master cylinder catalogs they dont differentiate?

Re: difference between brake a clutch masters?

PostPosted: Fri Jul 15, 2011 8:16 pm
by oldelanman
mini64 wrote:I always thought brake master cylinders had the secondary seal and piston that acted as a valve to minimize back flow when the pedal was lifted off, so the pads would stay close to discs as they wore. Whereas clutch masters have the one simple seal and piston. Is this wrong? If not, then why is it when you look at master cylinder catalogs they dont differentiate?


The original clutch master cylinder and the single circuit brake master cylinder are virtually identical the only significant difference being the larger capacity of the fluid reservoir on the brake one. (the tandem brake m/cyl is slightly different) The workshop manual has exploded diagrams of both cylinders and in both cases there is a valve ahead of the piston with a seal which closes off the port to the fluid reservoir as the piston starts to move, without that valve the fluid would be pushed back into the reservoir and no pressure would be generated in the system. Maybe that valve is what you have in mind ?

The master cylinder does not control pad/disc clearance, this is achieved by the caliper piston seal design. This is how it was explained to me many years ago by a Girling Engineer??.As the brakes are applied fluid pressure behind the seal tries push it out of the caliper bore and deforms the lip of the seal very slightly, when this pressure is released the seal ?relaxes? and tends to pull the piston back into the caliper and so a minimal clearance is maintained between pad and disc. However, if the discs are distorted or the wheel bearings not adjusted correctly ?pad knock-back? will increase the clearance and lead to increased pedal travel.

Hope this helps.

Best regards,

Re: difference between brake a clutch masters?

PostPosted: Sat Jul 16, 2011 2:24 pm
by prezoom
You are thinking of the residual valve required for vehicles equipped with disc front and drum rear. Where the brake shoe return springs compress the slave cylinder and push fluid back into the master cylinder. Vehicles equipped with discs all the way around, do not need this valve. Although my other Girling equipped lump, using the same master cylinder and front calipers, but drums in the rear, does not have this valve.

Rob Walker
26-4889

Re: difference between brake a clutch masters?

PostPosted: Fri Jul 22, 2011 1:18 pm
by mini64
Spent a little time thinking about it and realized the error in my ways. What I was talking about is called the "valve seal" in the attached photo. Where I was wrong was in saying the clutch master doesnt have this. If it's a girling style it does. Afraid I've pulled apart too many hydraulic systems! This valve is needed in the girling style because of where the fluid inlet from the reservoir is in relation to the outlet. This valve closes off the inlet when we first push the pedal down, so the piston doesnt just squirt fluid into the reservoir instead of directing pressure to the brakes. Other styles of master have the inlet first, then outlet, along the length of the bore and the piston passes the inlet on initial pedal application, cutting off the port so pressure only goes to the outlet.

Thanks for everyone's help!

master.jpg and