93 gmc braking problem

I have a problem with my 93 GMC 1500 truck that I have not been able to solve. Everytime I press on the brakes, the truck pulls to the left. It only occurs when I brake. The truck has been looked at by 3 different mechanics. They were unable to diagnose the problem. Brake pad, shoes, and rotors are fine. Some work was done on it by a friend that alleviated the problem temporarily, but the problem came back. Is a new brake system needed? Any suggestions would be helpful.

I have a 1996 3/4 ton with the same type problem. Always pulls to the left when braking and only wears out the front left brake shoe. The back brakes do not seem to hold especially in panic stops. The dealer said that the front brakes do all of the stopping anyway. Anyone know of a cure?

If it pulls one way while braking, then that means that the other side is not working.
Check the piston, lines and caliper mounts.