Brake fluid moving btwn front and rear systems

[Lenardsimp]

First the pedal started feeling soft. The the rear wheels started locking up under hacd braking. When I bumped someone at the stop light, I figured I should stop ignoring that little dash light.

Turns out every time I step on the brake pedal, a significant amount of brake fluid travels from the front brake master cylinder reservoir (the big one, in back)into the rear MC reservoir (the little one).
Enough so that with about 4 times pressing the pedal to the stops (which also shouldn't be possible) the large one is empty and the small one is spilling over through the seal.

From what I've been able to gather, the only places where the front and rear lines come together is in the master cylinder itself and in the
pressure differential valve.
I assume that the seal in one of these must be bad, and is allowing fluid one way and not the other (someone said that they are "umbrella" shaped, so I guess that makes sense)

So, my question is, which of these two is more likely to leak (or is there a third possibility I haven't thought of), and how do I determine for sure where it's happening without taking the time and expense of replacing a potentially good part?

Thanks all

--------------------
Bright Orange 1983 F-250, 6.9L D, 4sp Manual

 

[hillbilly]

First the pedal started feeling soft. The the rear wheels started locking up under hacd braking. When I bumped someone at the stop light, I figured I should stop ignoring that little dash light.

Turns out every time I step on the brake pedal, a significant amount of brake fluid travels from the front brake master cylinder reservoir (the big one, in back)into the rear MC reservoir (the little one).
Enough so that with about 4 times pressing the pedal to the stops (which also shouldn't be possible) the large one is empty and the small one is spilling over through the seal.

From what I've been able to gather, the only places where the front and rear lines come together is in the master cylinder itself and in the
pressure differential valve.
I assume that the seal in one of these must be bad, and is allowing fluid one way and not the other (someone said that they are "umbrella" shaped, so I guess that makes sense)

So, my question is, which of these two is more likely to leak (or is there a third possibility I haven't thought of), and how do I determine for sure where it's happening without taking the time and expense of replacing a potentially good part?

Thanks all

--------------------
Bright Orange 1983 F-250, 6.9L D, 4sp Manual

may be a cross bies system try bleeding brakes. right rear left front left rear right front. might help you out. system is made to mantain front and rear braking in case of failur? from the mountain .hillbilly

 

[Lenardsimp]

may be a cross bies system try bleeding brakes. right rear left front left rear right front. might help you out. system is made to mantain front and rear braking in case of failur? from the mountain .hillbilly

Wow, that was a long time ago!
I got a responce on another forum, and ended up replacing the master cylinder, bleeding the brakes, and its been fine ever since.

But still, thanks for taking the time to respond!

 

[hillbilly]

may be a cross bies system try bleeding brakes. right rear left front left rear right front. might help you out. system is made to mantain front and rear braking in case of failur? from the mountain .hillbilly