Genie GS2668RT. Brake problems

Hi I recently bought a 2001 Genie 2668RT diesel model. I have since discovered that the brakes are stuck on as the machine was quite slow to drive. I’m an electrician so had a look at the electrical side of things which is all ok. A mechanic friend had a look at it and believes that the valve that the solenoid operates relating to the brakes is ok as it shoots back and forward when tested on the bench. He put a pressure gauge on the end of the brake pipe at one of the wheels but didn’t think that there was enough pressure to release the brake but didn’t know what the pressure should be as the manual that I got for the machine doesn’t show this. The manual hand pump release doesn’t appear to be putting up enough pressure to release it either. Does anyone know what the brake release pressure should be or think of anything else that could be causing the problem? Thanks