As per my suggestion in a post further up, did you try to drive the machine and look for signs of pressure in the 2 large hoses..? If the machine is jacked up off the ground that's even better and I suggest that if you have not done that yet do it before going any further.
Let's explain the "drain line" a bit first. Your drive motor is a hydraulic piston type and all motors of this type by design leak internally by a small amount. The "case drain" line is to take this leaked oil from inside the motor case back to the hydraulic tank.. It is very unlikely that the case drain line could get blocked, but it is a remote possibility. So if you remove the 2 small lines at the motor end and try to blow through them you should be able to blow through one and not the other. You should hear bubbling in the hydraulic tank when you do this. Try leaving that hose disconnected, start the machine and try to move it. See how much oil comes out of the connection on the motor. It should be a steady trickle, but if it's a flood then your travel motor has a major internal leak and is most likely goosed.
Having identified which is the case drain line by a process of elimination you know which line is the brake release. Re-connect the hose, start the machine and try to drive that track while someone watches (or holds) that hose close by the travel motor. It should be possible to see/feel it "pulse" with the pressure in it when the travel control is moved. That's is the pressure that is releasing the brake and is as near as you can get to see whether the brake is releasing or not. If you have something like a Porta-power pump maybe you could rig that up on the brake line and release the brakes manually.
NOTE: For every line you remove check carefully for signs of metal particles both in the line itself and also in the connector on the motor. The presence of lumps of metal could also be an indication of problems inside the motor.