[EricH]

What is an "air compressor valve"? Do you mean:
a) the air compressor itself?
or b) the brass pressure relief valve (PRV) that attaches to the compressor (or appropriate brass hardware)?
or c) some other valve in the pneumatic system, like the solenoid valves?

The answers to all of those will be found in the 2012 Manual; however, if you would like to get an idea of what was allowed in the past, start at page 25 of the 2011 Game Manual Section 4. (Note: Anything seen in the 2011 Manual does not apply in 2012 except where the rule is repeated in the 2012 Manual.)