Touchy Brakes- lock up

[2badgn]

I have a 1984 GMC Sierra 3500 2x4 single rear wheels. Last year, I had my Chevy dealer replace the brakes. They cut the drums & replaced the shoes. They also changed both front discs, calipers, brake pads and brake hoses.
Master Cylinder was also changed.

My dad and I usually drive around town but this weekend i took the truck on the highway.

The brakes are touchy. Sometimes they lock up. When on the highway & I apply the brakes, there is some vibration but then it will go away.

I was parking the truck and as I was reversing very slowly, 1 of the tires skidded. Sometmes when driving, it seems as if 1 of the wheels isnt catching when the brakes are applied & the truck will hop or bounce or sink- Its tough to describe. Front end sinks to right slighlty.

Im going to have the mechanic check it out but wanted to know for myself, what could be the problem.