Odd brake problem.

Some trailers around that time had abs issues that caused slow release of brakes and hopping when empty on wet or snowy surfaces.

 

Hand valve fully returned?

 

How did you normally repair those?

 

Foot, hand and possibly tractor protection valve problem would be bleeding air out of tractor service glad hand when disconnected. That is with parking brakes released while the problem is occurring.

 

Pull the supply line off the valve and check that air is flowing freely , then pull the fitting off the valve and check for debris. If it is doing it on both trailers it will be the lead trailer valve. I have seen that happen on gravel trains here in Michigan.

 

Good point. Low supply pressure, flow, can get the spring brakes starting to set.

 

Some of the old valves actually had a screen behind the supply fitting that would catch debris, pull the fitting clean out the dirt and good to go again

 

A brake that offends you one day way not the next. It needs to be checked when the problem is occurring.
Seems to be and a tee with a gauge are two different things.