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Master Cylinder question

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nevets

nevets

Jedi Knight
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I read somewhere that if the fluid in the master cylinder reservoir raises when the brake pedal is depressed it is an indication that the 'foot valve' seal is not working properly. When I depress the brake pedal the fluid is disturbed (sort of pulses) but I do not believe the level is raising. Is this normal? Should there be zero movement in the fluid when the brake pedal is depressed?

Thanks!
 
I think it was me that mentioned possible foot valve failure. I just had my GF hit the brakes on my BJ8 whilst I watched the fluid; beyond some disturbance from the car shaking a little when she hit the brakes I didn't notice any pulsing. Are you having to pump the brakes to get good pedal resistance? If so, and you think there's disturbance beyond movement in the car you probably have a failed foot valve. Happened to me; mine went out on a long road trip and I had to drive over 1,000 miles pumping the brakes a couple times for every application. Mine was pretty chewed-up; if you just have a small leak you might not see a big rise in fluid level.
 
I would think you'd get some disturbance just before the "foot valve" seats. As long as the brakes hold pressure I wouldn't worry about it. I don't know how one would bet chewed up however, its just a little puck that seals over the inlet hole under pressure.
 
Brinkerhoff said:
I would think you'd get some disturbance just before the "foot valve" seats. As long as the brakes hold pressure I wouldn't worry about it. I don't know how one would bet chewed up however, its just a little puck that seals over the inlet hole under pressure.
Click to expand...

The foot valve seal is spring-loaded against the seat at the inlet from the reservoir; i.e. it should be closed--and sealed--under normal operation. After braking, when you release the pedal, suction in the MC draws fluid from the reservoir--overcoming the spring and seal--then seals again when pedal travel has stopped, so I don't think there's any delay sealing the intake line when the brakes are applied. I stand ready to be corrected, however.

Again, if you're having trouble getting a firm pedal and you see some 'disturbance'--that can't be attributed to shaking--in the reservoir fluid when you apply brakes, you likely have a failed foot valve.
 
Since its a closed system once the fluid enters the m/c the foot valve needs to allow the outlet line pressure to equalize with the reservoir pressure at rest or your brakes would not release. It does this by not sealing at rest , pulled back slightly off its seat by the spring loaded rod as the piston returns to the beginning of its travel. Its a pressure valve only and closes the inlet port when the piston is moved, therefore it wouldn't be unusual for a slight disturbance in the reservoir fluid once the brakes are applied but just before the foot valve seals. If your brake pedal is firm and the brakes release pressure , its working ok. If it has failed your pedal would go to the floor.
 
Brinkerhoff said:
Since its a closed system once the fluid enters the m/c the foot valve needs to allow the outlet line pressure to equalize with the reservoir pressure at rest or your brakes would not release. It does this by not sealing at rest , pulled back slightly off its seat by the spring loaded rod as the piston returns to the beginning of its travel. Its a pressure valve only and closes the inlet port when the piston is moved, therefore it wouldn't be unusual for a slight disturbance in the reservoir fluid once the brakes are applied but just before the foot valve seals. If your brake pedal is firm and the brakes release pressure , its working ok. If it has failed your pedal would go to the floor.
Click to expand...

Makes sense. However, I doubt--again, ready to be corrected--that there is much of a gap between the foot valve and its seat when the pedal is released; i.e. it's not gonna push a whole lot of fluid back into the reservoir if it's functioning properly. The bigger the gap, the more slack in the pedal before pressure gets applied to the servo (or slave cylinders). When my foot valve failed, the pedal didn't go all the way to the floor, but I had to pump the pedal 2-3 times to get effective braking.

When mine failed, I ASSumed it was the servo, so I overhauled it and only dug into the MC because I was out of other ideas. Didn't hurt to go through the servo, as they seem to need it after 100K miles or so anyway.
 
I don't know if it were a measurable gap but it is a function of the suppleness ( ? ) of the seal to seal quickly under pressure , and it wouldn't take but a bit of pressure to interrupt the surface of the reservoir.
 
I guess it's down to how we define 'disturbance.' I didn't do the math, but I suspect it's a fraction of a milliliter in a reservoir and line that I'd guess is about 100-150ml volume (the lines from the reservoir are larger than the brake/clutch lines). Anyway, if there's a tsunami in your reservoir, or the level rises appreciably when the brakes are applied it's a good bet your foot valve has failed.
 
Thanks for the discussion. I will assume that if after bleeding, the pedal is firm and does not pump up, that my MC is functioning properly.
 
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