Compressors operate hot after several hours running and is normal which is a big reason why it has a fan in the cabinet. Pressure gauge should not read 0 when the diffusers are operating. Gauge should read at least 1/2 the depth of the deepest diffuser. Possibly the gauge is bad?

To check if the gauge is working properly,,, disconnect the airline/s from the pump. If there are two airlines disconnect both. Close one valve and the gradually slowly put your finger over the air gushing out the open outlet. As your finger gets close to stopping all the air escaping or VERY briefly stops the air flowing out the gauge should show the increased air pressure that you are creating. If the needle does not show the pressure increased due to your finger restricting the air flow the gauge is bad and tell the company to ship you an new gauge for free.

aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management