Working on a Genie Z45-22 that when I try to move it forwards or reverse it will move about 4-5 feet and then the brakes come on; both wheels, its 4 Wheel drive and gas powered. I had this thing up on jacks and had it working great after replacing the drive proportional under the rear cover (it had a crack in one of the ports and it was about $2,800). Worked fine in the air, two speed worked brakes released, set it down on the floor and now this, (and NO it can't stay up on jack stands it needs to go out and work). I've been fighting this for over a month this machine has had about 7 different issues, that are mostly fixed.
You need to put a gauge on it and check your hydraulic pressure at the manifold and see if what pressure you are getting out of the manifold and to the brake release circuit. That would be the first and most important step and will help narrow down if the problem is there or if you need to move down to the traction manifold or brakes themselves. The service manual will give you the testing port location and desired pressure.