I would start with the release valve itself. this is located under the cab floor on the back of the release lever.
It could be that this valve simply isn't releasing, Maybe through wear and tear or recent frost damage.
Before your do anything please ensure that the vehicle wheels are chocked and in gear ( Engine off ) and it cannot move, because if that brake comes off unexpectedly the vehicle could start to roll
1. Get the Pressure up .
2. Release the lever
3. Tip the cab
4. Start giving the valve mechanism a good sharp tap .
5. If this doesn't release then move backwards along the line doing the same to every valve on the way.
Another option is one of the valves in the 4 way safety valve is stuck.
This is usually located near the air tanks. its usually a way valve filling up your air circuits in the correct order. One of the last to fill will be the Parking Brake, so if this valve failed it could still provide air to the foot brake circuits but not the parking Brake .
Again take the appropriate safety precautions and give each valve a good tap.
If you can identify which of the 4 valves is the parking break you could crack the union on it a very small amount to see of its open and providing pressure This valve should look something like this
Good luck and please secure the vehicle first
Neil
