Air brake problem

My truck and trailer has new brakes and drums. It's been doing this for a while and seems to get worse or do it more often. My brakes a properly adjusted and did this before I replaced the brake shoes and drums. Also the tractors shoes seem to wear faster than the trailers. It's 91 pete 377 dump truck with a pup trailer. The problem when I apply the brakes, sometimes it will brake fine with around 10 psi on the air pressure gauge. Then on the next time I apply then it will use 40 psi. When it does that it feels like my brakes need adjustment. They are no air leaks at all and I thinks it's a valve some where but witch one.

 

Ok thanks . That's the third time someone tell me that