Master Cylinder Burbing

[hexafamily]

1972 C10 with factory power front disk brakes. All brake components have been replaced except booster and proportioning valve. My problem is fluid keep leaking out from the top front of the master cylinder cover. The previous master cylinder did the same thing, which is the reason is was replaced. I have noticed the front brakes are weak, and the rear tends to lock up easily, and under hard braking you can not get the front to lock up. Sometimes the brakes feel like coaster brakes when come to a stop at speed. Pedal is firm, but not hard.

Any ideas on whats happening?

[OhOneWS6]

Did you replace all 3 rubber lines as well? They tend to swell with time and can become completely blocked off. Usually a new cap and seal will remedy the leaking master top. Since you already replaced the master I'm not sure what to suggest on that.

[biggestjohn]

The proportioning valve regulates how much pressure goes to each set, but also how fast. I have seen the valves inside them stick, but I really can't explain the fluid leaking out of the cover. The only thing that makes sense to me is that fluid is escaping from the rear set to the front set in the valve.

Is the reservoir from one brake set loosing fluid, while the other gains fluid?