[cwl1979]

Carl,

The hwh system is a little different than what you are imaging. With the engine running, the pressure is limited by the governor or the safety pop offs on the tank.

With engine off, the situation gets a bit more complex. The 120v compressor is normally controlled by a Square  D well pump switch. Cut in about 70, cut out about 90. HOWEVER, if manually leveling or in AUTOLEVEL the HWH system sends a 12V signal to a relay box wired parallel to the Square D switch. The 12V signal closes relays which turn on the 120V compressor bypassing the pressure switch. A new Thomas compressor is capable over 150PSI. There is a device that looks like a pressure regulator on the line, but it is a fancy pressure relief. In my case, that device either failed or had been misadjusted. I left the system in autolevel and exploded a tag air bag. POW! There is NOT an electrical pressure switch in the HWH system to limit high pressure.

You are correct. Their is no pressure switch for the tag axle. the steers and drives should have them though. In the manual i linked below it should be on page 10. Unless I'm not reading that correctly.
  I'm not fond of airbags blowing up in my face either. I've done it more than once.