1985 FORD F150 brakes

[rootercycles]

I am having an issue with my old FORD work truck. I am completely baffled by a brake problem I have with my truck. I completely lost brakes a couple of days ago. I looked in the master cylinder I replaced this year, and found the front reservoir completely empty. I traced all of the lines back only to find all intact, without leaks. I bled all four brakes twice. I test drove the truck and the problem persisted.I thought that my brake booster couldbe bad and I replaced that along with a new master cylinder. I can pump the brake pedal while driving and the brakes work well after 3-4 full pumps. The minute I step off of the pedal, the system loses all pressure until I pump it again.I can't figure out where the brakes would be leaking if I can't find a leak within the lines or from the calipers or rear drum slave cylinders. I have been told that the proportioning valve may be the next place to look.I have worked on brakes for many years andcan't let this thing beat me by bringing it to a brake shop!Thanks for any help you can provide!

[FordinmyVeins]

well, your brake fluid has to be going somewhere. it can't just disappear. i don't see how the proportioning valve could cause the problem you describe. possibly the metering valve could be bad and that will cause a low squishy brake peddle as well as low or no vacuum in your booster. when you removed the booster was there any brake fluid in the diaphram? that could be where your leak was if not in the lines, master cylinders sometimes leak out the back into the booster. check your wheel cylinders in the rear drums, just use a flat blade screwdriver and pop a side of the rubber sealoff of the ends and see if its full of fluid there should'nt be any in there. if there is that indicates seeping. its not technically a leak until it actually gets past the seals. oh, one more thing... if your truck happens to a residual check valve for the rear brakes, which if a its a 85' it should'nt, but if thats the case your check valve may be malfunctioning and will explain the low pedal and good pedal after pumping. there are a lot of possiblities to your symptoms, way too many to list, check back and let me know if this helps or if its greek I'll try to explain better.