Truck and trailer brake questions

I'm attempting to diagnose by the symptoms of what you describe since I can't be there myself. The alternative would be to perform air pressure tests that would become quite involved and labor intensive. The foot valve shouldn't break the bank and it is highly suspected here.

 

When was the last time you replaced the brake chambers and/or the service valve on the trailer ?

I had issues with inconstant braking with my trailer, I replaced the service valve since it controls braking pressure it was original equipment, I'm stopping smoothly & consistently now.

 

Good deal. You might consider rigging up two dummy gladhands, one of them plugged to mate to your red hose and one with a pressure guage screwed into it for your blue hose. Connect the gladhands to the appropriate hoses, release the parking brakes, build up to full air pressure, then mash the service brake pedal and note the guage reading on the blue gladhand. Should be above 100 psi on full application.