Typically there is a valve plate assembly. One valve opens to let air in then closes on the compression stroke, the other (to the receiver) opens on the compression stroke and closes on the intake stroke. Ok so far, this is the obvious.
There is another valve that automatically releases pressure on the head of the unit each time the pressure switch stops the motor. This allows the motor to restart with zero head pressure. This is the hiss of air that you normally hear when the motor stops. This is the "bleeder valve" you are talking about. It always released air, just not the whole tank. I think your problem is with the valve plate in teh head.
Try this:
1) Turn compressor on and as you get to around 70psi, pull the plug.
2) Does it hold pressure. If no the bleeder switch is stuffed. If yes, leave unplugged and turn the compressor of using the switch. A small amount of air should released from the head. If all air is released, you "probably" have a problem with the valve plate in the head.
