1997 classic air brake problem

hello I have a 97 classic this is holding air pressure in the red trailer line once the valve has been applied. The tractor (yellow) valve and brakes work properly . The yellow hose(exhaust ) from dash valve is clear and the dash valves have been replaced and still I have same problem. The issue is that while the trailer valve is in the park position the air is not being exhausted and the trailer brakes are staying released when they should be in the parked position. Anyone had this happen or ideas ?

 

LoJackDatHo, Thanks for the input . I have checked the air flow coming from the red line, but when the brake valve is applied I however cannot tell if it exhausts immediately because I'm in the cab I will get someone to help, however I have hooked to two different trailers and still have the same problem. I was wondering where does the air disperse exactly when the valve is pulled out?

 

I will check into that first thing in the morning. Thank you

 

Thanks for helping, the tractor (yellow) is exhausting fine from yellow exhaust hose . So I know it's not plugged and valve is new and is doing same as old valve . I did remove glad hand from trailer and worked trailer valve and it seems to work properly , its only when it's hooked up and brakes released by the valve and then placed in park position that no air is exhausted on the trailer side only. If you remove red air line a burst of air comes out and trailer brakes will then apply

 

So I checked trailer and the last valve so I thought is the tractor protection valve , which is working as it should . I turned my attention to the two valves attached to the trailer air lines as they leave the back of the cab and found that the emergency one was recently replaced , but with a regular check valve from napa and not from dealer . Removed and replaced it and long behold all is working well.