I have a restored original 85 gmc brig tractor 6-71 n ten speed Ouestion is that truck cannot charge trailer unless tractor button is in also. Is this normal for this year truck and if it is,can it be changed to just be able to have in the red button to charge the trailer, and still keep the tractor brakes on . most of what I am doing is with a air ride trailer, and I have to chock the wheels so rig does not run away when trailer is charged. thanks Al.
Does this truck have the 3rd blue button in it?
Can you get a photo of the dash valves, If it is the early MV3 two piece white plastic type, there was a part number in that group that was internally plumed that way. You can change it to the later MV3. If it has separate PP1 valves you can split the charge line so each valve has it's own supply.
The blue is normally for emergency release, depending on how it was plumbed.
That would be a pain if plumbed that way. My tilt takes a while to get charged up.
on the firewall, it has two square valves piped into the foot valve, just two buttons on dash red and yellow ,
Look at the back side of valve and see if you can snap a photo. If it is white you need to up grade it anyway, those were bolt together and the plastic would warp over time causing internal leakage.
I thank your truck is like Truck Shop said in post #3. Get the trailer park valve its own air supply instead of being feed from the tractor park valve when released.
Not that the cops are checking, but on these old style systems I thought that the trailer valve was supposed to be supplied from the tractor valve so as to set up automatically if the tractor valve was pulled. And the third (blue) valve was added for trailer charging, if desired. And the MV3 takes care of that problem but requires its big blocky self to be installed (fit) somewhere.
Your correct BV but not every system was plumbed the same in those days of single valves. The reason for MV3. Mack had some from factory in late 60's tractor valve was in the dash and trailer valve mounted between the seats, flip type PP1.
Bendix used to have easy to follow for the layman charts. Remember seeing them in the stock room at my father's work years ago.
And there it is