This is not a common problem, but the first thing I would consider is the combination valve. That is a brass block that sits on the frame rail right under the master cylinder. It will have a single wire plugged into it in the middle, on top. Follow the steel lines from the master cylinder to this valve assembly.
The front valve is the "metering" valve, also called the "hold-off" valve. Its job is to require the brake fluid to reach about fifteen pounds of pressure to open that valve so fluid can flow to the front calipers. That is to give the rear shoes time to expand and make contact with the drums. It insures front and rear brakes start to apply at the same time. On some vehicles, older Fords in particular, that metering valve has to be held open by hand or with a little special spring-metal tool to allow for gravity-bleeding of the front brakes. When that tool is forgotten and is left in place, you'll get the real easy front-wheel lockup you described.
The stem of that valve sticks out in the front of the assembly and it might have a rubber cap around it. You should see that stem pop out a little further when a helper presses the brake pedal.
Has the ride height been altered in the rear or is it sitting higher than it is supposed to be?
Monday, September 19th, 2016 AT 6:48 PM