Could be damaged brake shoes or hardware assuming they are drum brakes. Remove rear wheels and drums for a brake inspection. This should show you what's going on. The year, make and model info would help.
does you truck have drum brakes in the back
Slam on your brakes. If your brakes lock up, then you do not have ABS. If your brakes do not lock up and you hear a knocking type of noise from your brakes, then you have ABS.
Most likely you do not have anti-lock brakes in your vehicle. Other possible answer is that your anti-lock brakes are malfunctioning. If you have anti-lock brakes you should see a mechanic as soon as possible.
The Anti-Lock brakes pulsate so the brakes do not lock up (anti) and cause the vehicle to side out.
Assuming they are Drum brakes most likely the wheel cylinder is bad.
The abs brakes on the back of a 94 S10 are designed to help slow the truck down quickly. Without abs, the brakes can lock up and the truck could slide uncontrollably.
There are a couple of things that can cause your Pontiac antilock brakes to lock up. The most common cause is a malfunctioning ABS sensor.
The dealer will know how to access the codes for the brakes. When that is fixed it should not lock up then.
It is possible for the brakes to lock up on a 2000 Chevy Venture when in reverse because the brakes are not working properly. Perhaps the brake pads need to be changed.
Lets say you do not have ABS. If that is the case, is it the front or the back brakes. If it is the back brakes, then it is possible that the brake springs have to be changed, that is of course if you have drum brakes. If you have 4 wheel disk, then its another problem, possible the brake equilizer valve has to be replaced.
When your brakes get harder and harder the more you use them and lock up, this is a symptom of brake failure. You need to take your vehicle to a qualified mechanic.
if back brakes are drum i would think they are worn out not allowing pistons in wheel cylanders to return. also could be valve in master cylander.altho that would cause all brakes to lock up. if disk brakes your calipers need overhauling or replacing