There is not a check valve or anything. There is a solenoid in the head of the compressor that releases pressure to lower the shocks. Look for 2 wires (Orange & white) going into the head from the top. Those are the solenoid wires. One should have +12v (Orange) from the same fuse that runs the compressor and the other (white) gets a ground directly from the sensor. So I guess that would be a good place to start.
Check to see if you have 12v on the orange wire. If so, ground the white wire at the sensor and see if the solenoid works and releases pressure. That will check out the wiring all the way to the front. Of course, if it works, it is either the sensor or the loss of its ground at the black wire. BTW, it uses the same ground point as the inflate switch circuit in the trunk if you have one.
All this is for a Bonneville, so I have to guess yours is the same. Let us know back what you find.
1993 Red SSE - Bone-Stock except for High Perf. Power Antenna-Original Owner. Less than 130K - Love of my Wife
'05 Magnum R/T Hemi - Bunches of classic VW Beetles... and one "not quite stock."