Brakes, nothing until bottom, then lock

[farmallboy15]

I'm currently looking at buying an 85 k3500 dump truck. I took it on a short test drive and thought i was going to die...

Hydroboost system:

When you press in the brake pedal, nothing happens until the very bottom of the travel. At that point, the rear brakes lock up. Nothing from the front by the feel of it. The top travel where it flops around, you get some random back pressure, feels like the booster. The idiot light is not on.

My thoughts: large air bubble in the lines, proportioning valve needs reentered and rebled. Or a bad master cylinder.

Any and all ideas are appreciated!