IIRC I Have heard there is an internal bypass valve in the MCs to avoid damage to the piston cups if they are over pressured. Not fully sure of,nor completely informed on this. The VacBooster increases the force on the MC pistons (multiplies the applied foot force) many times over the non boosted (engine off) pressure, and at some point the pressure could be too high for the piston seals to seal resulting in bypass, and fluid loss back to the reservoir. If the MC bore is too large or the piston seals too small this could be happening to us, just like a worn out MC, where the bore and cup seals are too worn to hold enough pressure. If this is the case, where do we find good Jeep Renix era MCs now???? I have tried what I was told was the best brand, Raybestos from Rock Auto after giving up on 3 Autozone brand new MCs.
And how do we prove it is the cause first? I guess if I knew the peak design pressure for the brakes I could install an inline HP-hydraulic gauge, about 10,000 Psig on the lines somewhere. I know it feels like my MC is not holding peak pressure, but just enough to have near-decent brakes. If I press the peddle fast enough at 60 mph I can toss an elephant through the window, but not when applied slowly to the bottom of the stroke, making me think all four new MCs are leaking internally.