Master Cylinder Leaking

I have a 1977 F-250 Dually, when using the brakes the front well on the mastercylinder pumps out from under the master cap. I replaced the master with a new one and it still empties the front well from the cap, there are no leaking wheel cylinders or lines. What the hell is going on? This is driving me crazy!
Neil[img]/forums/images/smiles/icon_confused.gif[/img]

I replaced the mastercylinder and checked the wheel cylinders for leaks, it is still emptying the front well on the master cylinder. Could this be a proportioning valve problem or a bad power booster? The truck has a dually rear end in it.[img]/forums/images/smiles/icon_confused.gif[/img]

How are the brakes when you drive?
Feel ok, or like a sponge?
If it's dumping the secondary {front part on the master cylinder}, you need to crawl under the truck and look the lines from front to back.
Does the warning light come on at all when hitting the brakes?
The differential valve might have shifted inside the prop vavle if there was a pressure on the back line, but, well, not sure.

I had a bad master once, on my '84 f150, was dumping fluid into the brake booster.

Hope this helps some.
[addsig]

The pedal feels normal, no leaks under the truck or at the wheel cylinders. If the differential valve needs to be re-centered , how is this done?

<font size=-1>[ This Message was edited by: Neil74 on 5/11/07 11:14am ]</font>