This is on a completely rebuilt(new) car.I was sitting in a driveway waiting to pull out and brakes went way low and the brake light came on.Checked master and rear was low,had a slight leak in back that I had fixed so I just topped it off.Now it seems to be building so much pressure it's seems to be pulling fluid from the rear and pushing it out the cap in front.Just checked it again and after I drove it the rear is down and the front is overfull? Any ideas? Thanks, Mike
Car has 1500 miles and it's been fine up to now.

Never heard of that happening before.

I wonder if the rear piston in the master cylinder is leaking and forcing fluid into the front of the master cylinder. The only other place fluid could cross from the rear system to the front would be at the prop. valve. I suppose the pressure sensor in there for the brake warning light could leak, but at that point I would think both sides would be equal pressure.

I'd look at the MC.

Tim.

Sounds like an internal problem with the Master cyl.

Thanks guys, everyone I talked to is leaning toward master cylinder also.Thought maybe prop. valve but that would be too easy!!! Mike