M543A2

The truck does not need to be connected to the trailer in any way to release the brakes. If it was sitting with the hand brakes set, two things are possible. The shoes might have stuck to the drums or the cables from the hand levers to the backing plates are not sliding to release. Yes, the levers up is the released position. Work the levers and see if the cables under the trailer move in and out of the housings. Try flexing the housings with your hands to get the inner cables to move in the housings.
If the levers were not set, the shoes might have been adjusted so close to the drums they stuck even though the brakes were not set. For stuck shoes, first try hitting the backing plates with a heavy hammer to jar the shoes loose. You can also back off the adjusters to get shoe clearance.
If none of that works, you will need to try to get the hubs and drums off by taking off the wheels and removing the outer hub bearing. You can then possibly get enough movement of the hub and drum assembly to break loose the shoes. I would then go ahead and remove the hub and drum to see if the shoe anchors are sticking.
Regards Marti

Warsaw, Indiana
3 M-880 Pickups
2 M-200 Trailers
3 M-105 Trailers
2 M35A2 Deuces
1 M-139 5 Ton Bridge Truck
1 M-543A2 5 Ton Wrecker
2 M-51A2 5 Ton Dump Trucks
1 M-52A2 5 Ton Tractor
3 M-135/ M-211 Deuces
1 IHC T-9 SeaBees Dozer
1 M-313 Shop Van
1 M-689 Shelter Dolly Set
1 M-715 1964 Jeep