Trailer brakes locking up issue

Having an issue with the trailer brakes being very touchy, they seem to apply pressure sooner than the tractor. Not so much of a problem when loaded but empty I'm burning rubber often times causing tires to flat spot which causes vibration.
Anyway, I've talked to someone who has told me that it is a proportioning valve on the truck but I'm not sure of its location. Im asking because I got 2 different takes on the problem.
1st was a modular value underneath the 5th wheel
2nd was a valve located at back side of the cab, driver side where the trailer parking and service air hoses are screwed in.
Anyone knows for sure which one is it, Im trying to avoid throwing parts at it since its either hard to find them or they are expensive. Appreciate your help.

 

It's a 2021 MTM with wabco brakes. It most likely isn't the trailer as I had another tru k hooked up to it and it didn't exhibit same behavior. All I can tell you is that I've noticed that brakes work fine when they're cold for maybe few minutes. The cooler weather the better they work but only for short periods of time. Trailer had just brakes redone including shoes, drums, slacks, and all chambers replaced and djusted angles required by slacks to work properly.
As far as truck goes, New shoes, drums, new camshafts, cam bushings, slacks and chambers. I'm inclined to believe it's a valve issue because brakes behave differently in different outside temperatures. When temps are over 25 degrees Im experiencing issue from the get go.

 

Thanks for heads up, Looking into it and will update soon