Can a master cylinder cause too much pressure

[Welder Dave]

I have new clutch master cylinder, rebuilt booster and now a new slave cylinder. The slave cylinder is working but appears to have a slight weep. I'm not sure if it's normal for a very small amount of brake fluid to leak past the cup seal. I wouldn't think so but maybe it will wear in. Another thought is if the slave isn't adjusted right the master cylinder will be will be pushing the piston/seal too far and it weeps? I filled the master cylinder full but wondering if too much fluid could cause too much pressure on the seal?

 

[1693TA]

Short answer is no, the slave cup seal should not leak and must be replaced if it is leaking, or bypassing fluid. Usually a "weep" is an external leak and this is usually a sealing surface, or seal itself being imperfect. I don't know if in the application the seal in question for the end is a simple "O" ring, or a lip seal/packing of some type fitting into a gland. If the slave cylinder bottoms in it's bore or the release bearing/throwout fork bottoms/binds against something a cup seal is going to rupture, or something is going to break and/or bend.

I've never tested a hydraulic clutch release apparatus myself but hydraulic brake systems usually develop no more that 1200psi with 350, to 500psi being normal in a " panic stop" just before wheel end brake lockup. This would be for automotive braking systems. To get 1200psi, you would be literally "standing" on the pedal.

 

[56wrench]

If the rod coming out of the slave cyl is adjustable, you need to find the correct measurement/adjustment for your particular machine. And, as 1693TA said, no it shouldn’t weep.

 

[Welder Dave]

I was reading a service manual and I think I have too much freeplay. I did thread the eye bolts on the slave further on than the old one. I think it only weeps at full extension.

 

[Welder Dave]

I read something else that said there needs to be pressure on the seal when the piston is retracting so having too much freeplay may be causing a slight weep when it's fully retracted or the split second before there's pressure when you push the clutch pedal. I'll adjust the slave eye bolts longer and see if it helps. Knowing the slave is new and the bore should be good I'm not opposed to putting a new cup seal in now that I know how simple it is. I'm 99.9% sure that welding the proper ports on isn't causing the problem but it could be possible the original holes for the ports are too far down. When the piston is fully retracted the cup seal lip is is partly over the hole. I've seen other slaves where there's a plastic piece above the cup seal that when pressure is applied it pushes on the cup seal lip to insure a better seal. It has a matching bevel to fit the cup seal. Something like that might be available or not difficult to make. I'll see if I can find a pic. There is a spring on the fluid side of the cup seal but it is pretty weak. Maybe a spacer or stronger spring would help the seal??