Brakes do not have the stopping power

This is my son's truck and his friends have been working on the front and rear brake pads, and did not want me involved. The brakes now are not working properly and I am elected to figure it out. I see no leaks. There is good pressure for the front brakes. There is weak pressure for the rear brakes. A lot of pedal pumps and the brakes will stop the vehicle. I am thinking that a rear brake pad piston is stuck, or the rear pads not installed correctly. My plan is to raise a wheel up and while spinning it press the pedal and see if it grabs. I somehow suspect the rear wheels. Also, looking in the brake reservoir the fluid looked like motor oil and not brake fluid. No telling what oil was put in it by the friends. I was looking for some comments about my situation. Thank you so much for any help. Randall B, Pickford, Michigan