What do you use the trolley valve for?

Maybe cause a problem if you pull it back full pressure with the spring brakes set. Just like pushing the foot valve with park brakes set. It may compound them in that case.

 

Or it was left partially applied?

 

30 years ago yes. Air systems are now engineered with anti compound valves.

 

If that happens on modern equipment, something else is wrong. It the same as pushing brake pedal. Only thing different in trailer is valve crack pressure. Trailer applies before truck. Stops rear to front. More than likely you will never notice this happening.

 

Interesting. So what typically causes them to be out of adjustment?

 

Worn out parts

 

Brakes are a wear item, and need regular inspections and maintenance.
Every time you use the brakes, the shoes or pads wear down, and to a lesser extent so do the drums, and discs.
But other parts also wear out, bushings, S cams, pots, slack adjustors, and more.

 

It's your load, so your brakes are going to stop it. Not mine.

At least that was our attitude in the old days....

 

Use, mainly. But with automatic slack adjusters all you need to do is put the brake pedal to the floor with at least 100 psi and they should adjust themselves.

 

Can you hook the red line up to the blue gladhand temporarily to check the service brakes and check for leaks? I was thinking the constant pressure would activate the service brakes.