Brake fluid lose thru booster?

[Cayoterun]

On Thumper's truck, I'm using '84 GMC brake booster and master cylinder.
The booster vacuum line is run to manifold vacuum at base of carb.
The master cylinder is a new/rebuilt.

Problem: The brakes work great. But somehow, after driving the truck a short time, it will draw the brake fluid from the small compartment (rear) of the master cylinder back thru the booster vacuum line to the manifold.
Front compartment, (large), stays full.

Is the master cylinder bad, or is the booster messed up. I used the booster from the donor truck, and know no history on it.

I used General discussion as I didn't see a brake sub-forum here yet.

Any help appreciated,

[TimC]

your master cylinder is bad. If you don't replace it, it will also take out your booster over time.

[Cayoterun]

your master cylinder is bad. If you don't replace it, it will also take out your booster over time.

Master cylinder will get changed tomorrow. It hasn't had much use, until lately, but has been on there a couple of years.

Isn't this new forum great! Thanks to Jeanie and all others who set it up.



Thanks, TimC