M 931 A2 Tractor and 50 ish foot fuel trailer conumdrum

Heres a head scratcher, iam at a stop here. Recovering a 2014 rebuild 931A2 tractor and attached fuel tanker, Tanker is new never even had fuel in it Tractor has set since 2016 with the trailer attached , Got it running, wasn't too bad! runs nice, pulled tractor and trailer ahead 100 feet or so, all indications were good, Trailer took air well Wheels released and came forward with ease. Let it idle for half hour or so, Was recovering another M 931 with attached flat bed trailer as well.

went to pull out with fuel tanker and trailer brakes are locked tight all tires. Checked all the normal culprits. Air pressure in cab is good, Air load is applied to trailer, Trolly brake handle is UP, no leaks on either side of brakes, Disconnected checked each connection point no leaks , good air! Loaded, unloaded air on trailer valve numerous times, Depressed brake pedal , tried every thing I could think of. Has to be a valve. ANY IDEAS GUYS. thanks up front for your time and experience

There is a safety air switch on the fuel door were the hoses are on the left side. If the door is open or the switch is bad the trailer brakes will come on.

Thanks!!!!! good thought The fella who was with me had everything open ! ill check it tomorrow head back early its a three hour one way, To figure it out and get it moved

A big thank YOU to M 35A2 AZ.. you were spot on, The air valve at the hose door wasn't getting any air and wouldn't allow the spring brakes to release. So I back fed to the tank thru the schrader valve below the air load valve and after purging the tanks , lubing everything and exercising the valve and spring brakes it let loose and pumped up to work ! thank you again ! all fixed , all delivered, hope the new owners love them both !

Glad you got it going!!