Clutch master cylinder

Is it possible to have a master cylinder that sucks air past the seals, but no fluid loss? I rebuilt the clutch slave due to fluid leakage, but the pedal goes soft after about 25 strokes. No fluid leaks anywhere now and I've bled the system multiple times.
 
Sounds to me like the piston is sticking in the bore of the master cylinder. Have had this happen on new brake master cylinders from tilton . Did you build the unit up dry? I have also seen people use a small touch of grease on the seal in the cylinder and that seems to make it worse. I usually apply a small amount of brake fluid on the seal before assembly and never had any problems.

Hope this helps
 
Is it possible to have a master cylinder that sucks air past the seals, but no fluid loss?
Dave

This is virtually impossible. If air is getting in, fluid is getting out. When you press the pedal, the pressure in the system may be 2K or 3K psi, but on pedal release, there is not a corresponding high negative pressure to suck air in. That's sort of the basis of the mechanical advantage in a hydraulic system: small force (pedal pressure) on a small piston creates great pressure with little resulting movement. On the return (pedal up), very little pressure is required on the small piston to create great movement (pedal movement).

From what you described, I'd be willing to bet you have an internal seal leaking or a scored piston/bore. Somehow, pressure is leaking internally. Some seals are one-way, meaning there is a right way and a wrong way to install them. Also, it can be quite easy to tear a seal when you're installing it and not know it. Since you are not experiencing external fluid leaks, it has to be an internal pressure leak.

I have also seen people use a small touch of grease on the seal in the cylinder and that seems to make it worse. I usually apply a small amount of brake fluid on the seal before assembly and never had any problems.

Never use grease to lube the seals in a hydraulic clutch or brake system. Always use the native fluid (brake fluid).

Hope we have you pointed in the right direction.

Eric
 
I was not recommending using grease , I was just saying that some people use that wrongly and it causes more problems .

Hope this clears it up.

Cheers
 
Robin

Sorry if I made it sound like you were recommending grease. That was not my intention. You clearly stated that using grease caused more problems. My apologies.

Eric
 
Never use grease to lube the seals in a hydraulic clutch or brake system. Always use the native fluid (brake fluid).

You should never use conventional grease, true.

However, brake assembly lube is specifically designed to lubricate the rubber parts in braking and clutch hydraulic systems during overhaul.

Here's Wilwood's offering:

Wilwood High-Performance Disc Brakes - Brake Fluid & Assembly Lube

Here is PBR's stuff:

Pegasus - Assembly Grease for Brake Components


Permatex, ATE and other makers also offer similar products.
 
I just rebuilt one hydraulic clutch system.
Tried to get air out 10-15 times (with manufacturers instructions), pedal was still too soft. Solution was to do it from another direction; ie. I fed fluid from slave to master. One shot and it`s ok now.
 
Thanks for the replies. Finally located problem with some fluid leakage from McLeod slave cylinder. This after I replaced the seals! Yes I assembled with brake fluid. The slave is not out of the car yet so not sure which seal failed. I think I have the later McLeod design with 90 degree fittings, but they still have single rubber seals. They are not what I would call banjo fittings. I tried to make sure the lines were not distorting the seals. Anyway they're going back to McLeod after only 6000 miles of use. I can see the advantage of external slave cylinders, but I never had this kind of problem with other internal slaves on formula race cars I owned.
 
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