My van is losing assist when braking. New master cylinder, and power booster. Don’t know anything about the porportioning valve but was told it is working properly. But buddy thought my lines were hooked up backwards to the master cylinder, not saying this is probably the problem but was curious. Any help is greatly appreciated! Gonna try to attach photos of mine.
I believe your buddy is correct. Larger chamber should supply the front, for the stopping power. But larger line for the length of the rear line. Not 100% but 98 or so. Good luck