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It suddenly dawned on me that maybe this why we have heard several reports over the years of deliberatly plugged "relief" ports and filler caps without vents. Perhaps it was an effort by some rebiilder et-al to contain the fluid UNDER PRESSURE should the compensator valve hang open due to dedris. Thus providing some rezidual braking effect that would otherwise result in zero brake effect.

****** EDIT ******
Note that i AM NOT presnting the above conjecture as a viable, sound eng'ring modification. It is ONLY presented as a possible explanation for a shade tree kind of gerry rig fix to prevent TOTAL AND COMPLETE LOSS of brakeing effect shoild the compensator valve malfunction. I make no recommendation for nor against the procedure. I only present it as an explanation to WHY such procedures HAVE shown up over the years.

The compensator valve has no communication with the relief port. Let me try to get this across one more time in another way: The relief port is only a mediating safety feature: if the fluid seal starts to leak the fluid can find its way back to the reservoir. If the vacuum seal starts to leak, the port allows air to be drawn in from above the fluid level.

If both the fluid seal and the vacuum seal leak, then fluid will will be drawn into the vacuum side and hence into the engine. That would be the "white smoke and dropping fluid level" failure mode.

When the compensator valve "sticks", or more accurately is held open by a blob of dirt or coagulated braked fluid, all the pressure generated by depressing the pedal goes woosh back into the reservoir instead of stopping the car. This is the "pedal suddenly goes to the floor" failure mode. The relief port has nothing to do with it, and plugging the port will not change that type of failure. The compensator valve must seal tight each and every time the brakes are applied, or they will simply not apply.

SO, the failure of these units comes either through "normal" seal failure as on would see in any hydraulic system, or it comes through the compensator valve not seating. That is the particular achilles heel of the Bendix Treadlevac, and that is best mediated by careful filling and occasional changing of the brake fluid.

Quote:
" When the compensator valve "sticks", or
more accurately is held open by a blob of
dirt or coagulated braked fluid, all the
pressure generated by depressing the
pedal goes woosh back into the reservoir
instead of stopping the car"

Yes. Does that not also mean that the reservoir thus fills up with fluid????
Since the "relief" port is open to the reservoir then the excess fluid is pushed into and down the relief port as weell as maybe out thru the vent in the filler cap.

I edited my post #91 above.

Yeah, if the reservoir was full to the brim you might build up a little pressure till it whizzed out the filler cap vent.

But not down the relief port too?!???

On second thot:
The outer seal, that is the vacuum seal relies on negative pressure (vacuum) on the power unit side as well as positive pressure on the side that subtends with the bottom of the relief port. So then any pressure due to fluid forced down the relief port would only tend to increase the.sealing effect of the vacuum seal.
So any smoke observed then would have to be due to a bad vacuum seal.

When i get a chance i'll try to calculate the max amount of.fluid displacement made by the piston. Calculate the volumn of the reservoir.

Standard brakes are very simple.

Haha Amen. But there is nothing to discuss!i

Quote:

That's very thorough, Ross, and explains a lot. As I mentioned before, the white smoke, I think, is due to two things failing (or compromised) at the same time. I believe your second paragraph states that exact thing. The compensator valve failing BY ITSELF would not create the smoke, however, as you state, it would also lead to "pedal-to-the-floor" brake failure because it only happens when the compensator sticks open. I believe both conditions have happened to my particular unit at different times. Sometimes the brakes fail and you see smoke, sometimes, apparently, the vaccum seal holds, and it doesn't smoke, even though the compensator valve must be stuck open.

The gist of what I get from your post is, a stuck compensator valve is ALWAYS part of the failure condition. Leaking seals will only result in brake fluid loss, UNTIL the compensator valve sticks-- then, there may or may not be smoking from the tailpipe.

I wonder if the smoke results when enough fluid has accumulated in the vacuum section to reach the vacuum port at the top of the vacuum piston after the pedal is released. Again, I think that the mischief that creates the "sucking" and "smoking" situation happens after the pedal is released.

The biggest culprit in this whole thing is that compensator valve.

Thanks a lot for explaining that the FLUID SEAL gets some "help" from the relief port when the seal is failing, and differentiating the fluid seal from the VACUUM SEAL. I was under the impression that one seal was serving between the fluid and the vacuum sides, but apparently this is not correct.

I have asked my mechanic to save all parts as he tears down the MC, and let me photograph them before he starts the rebuild. This will also give me a chance to check for pitting around the seal.

My question: is the pitting only found only around the fluid seal, or can there be pitting around the vacuum seal as well?