I have a '78 E-150 stretch that I use as my mobile service truck. After I bought it, I put in 3/4 ton I beams, front springs and spring perches, 3/4 ton diff w/3:11 mid-west gears(1/2" studs), 3" 3/4 ton leafs, new perches and an air bag system. THEN after a couple of years of too high a gears, I installed a 3/4 ton diff with 9/16 wheel studs, 3:56 gears and removeable drums. also changed out the front I-beams to match (1985 I think). ANYWAY, when it is slick on the roads or even wet, the front brakes lock up and skid while it feels as though the rears aren't even working....I have to stop with my left foot on the e-brake, my left hand on the release, and my right foot on the brake pedal for the fronts. the brakes are all 100% new at the wheels, and adjusted up just fine, also got lots of e-brake. I'm planning to put a master cylinder from a '85 on there to see if it helps......any opinions or personal experiences?

 

Make sure the brakes are fully bled, all fittings are tight, and that the brake hoses are in good shape.
You might also take a look at the proportioning valve, and the master cylinder should be changed to the correct one also.

 

What type of brakes do you have front and back? Ford_Six is probably on to something. You also may have front discs and rear drums. Based on the description, seems like the rear brakes are the issue.