I have a brake problem with my 83 f150. The front brakes grab while the rear ones do little braking or none at all. I have recently replaced a front rotor, new front pads, one rear cylinder and rear shoes. The system has been bled several times with no improvement. Yet in 4 wheel the rear appear to work. This has been an on going problem even before the work previously mentioned was done. Could it be the master, the porportioner, brake lines plugged or what. It is a real problem when trying to stop on ice.

 

The rear brakes should only contribute about 20-30% of the total braking force for your truck unless you are fully loaded. The front brakes are oversized to handle a larger proportion of the work. So, I think your rear brakes are fine and the front ones just need some time to bed in to prevent the grabbing. The last two ford trucks I have owned have always been a little grabby when I first drive them. The more braking you do usually clears it up. If you really think the fronts aren't doing there share of the braking and you have totally replaced all the fluid, I would recommend replacing the Master cylinder. What could be happening is the front brake chamber is bypassing the internal o-ring and pressurizing the rear brake chamber. This is a normal failure mode and allows braking force to be applied even if the seal is bad.

As for the brakes working better in 4wd, I would guess that the additional engine braking you get to all 4 wheels is actually what gives the feeling of "better braking". It's a guess though.

BTW - If you have anti-lock brakes, the diagnosis gets quite a bit more complicated.

Good luck.
Randy