Brake Problems- 72 3/4 ton 4x4

[71 super]

My little rother has a 72 3/4 ton 4x4 he uses to pull his boat with. Loads boat up two weeks ago and starts out, leaving a fluid line out the driveway. No visable leaks. No cracked lines no expanded hoses etc. but rear reservior is empty (controls the front brakes) We replaced m/c today since it is realativly cheap and easy. We got some brake back but the dash light is still on and low pedal. Cylinder was bench bled and brakes were bled after installed on the truck, but brakes are weak at best. Any ideas I am leaning toward prop valve issue - reset or ??? or diaphram problem with the booster. Any ideas or suggestions are appreciated. How do you reset a prop valve to get light to turn off also would help. Thanks Harold