Trailer air supply line pressure? I have a supply problem

I've had an occasional problem for awhile with my airbags and sliding tandem not working all the time, slow to build pressure. I replaced pressure protection valve and leveling valve in trailer today but still have a problem. I am getting pressure to everything in the trailer but not enough for everything to work reliably.

Any idea what the pressure coming from the airlines at glad hands should be? I'm going to rig up a gauge to a glad hand tomorrow morning and test what I've got.

I'm thinking the big valve on the firewall may be bad. I don't know what it's called. Has big green and red lines coming out of it, the green one goes straight under cab to trailer air line manifold at back wall. I'm thinking it's bad because if I supply air to the trailer and pull my red air line it just hisses, it does not pop closed on the dash as it should.

 

That's the valve I am thinking of but he says that is the foot valve. I have no problem at all with brakes, I have a problem with air supply to the other things in the trailer (tandem release, tire inflation and airbags). I have replaced all valves in the trailer and checked and double checked them.

Oh, by the way, I don't have any backfeed coming out of the foot valve like in the video.

Any idea what valve I should check next?

Also, I don't have a spare truck or trailer. I'd do both if I did, connect another truck to my trailer and another trailer to my truck.

 

Just replaced and chased all that down yesterday. I am not getting enough pressure to supply that regulator/regulators. Or I should say I am but only intermittently.

 

@Ridgeline, thanks, I went over and over the trailer yesterday. I've got to rig up a gauge to test air pressure at glad hands. I do know that I might have two problems. If I supply air to the trailer (red knob in) and yank it loose my red knob on dash does not pop, just constant rush of air from red air line. I know this is a problem, I just don't know if it's the only problem.

I'm going to rig up a gauge this morning. A friend also suggested taking my air compressor to the trailer and supplying trailer with it to see if everything there is working.

 

And also, what should supply line pressure at glad hands be, 120?