Pretty sure that, ultimately, regardless of who made it, they work the same way. Motor turns on, air pressure builds to a specific PSI, braking motion causes the air pressure to activate a cylinder that presses the brake pedal.
My initial thought is that there is probably a seal gone bad in/around the air tank so you can't build pressure, considering you're saying that the motor does run. You might be able to luck into someone who can rebuild your typical air compressor who can give it a lookover. For all intents and purposes, that's pretty much what it is, just with other bells and whistles that make it work under braking.
