Air Pressure Drops When Ignition OFF

What I have done before is track the leak down my self and then get the shop to fix it... They don't so much mind the fixing but some are to lazy to do the finding...soapy water in a spray bottle helps to track it down if its not audible

 

So it's the tractor. Have you checked leveling valve? or firewall quick connect?

 

Does a ProStar have both a primary & secondary pressure gauge on the instrument cluster?

To your quote above - it certainly seems like a tractor issue. But after the secondary is gone it'll leech the primary.

You work for Central Transport, per chance? Oldest piss-yellow equipment in the game? Lol.

 

There is no leakage rate specified in the Regs which would make it illegal. If the compressor can outrun the leak at operating rpm and there is no audible leak, it’s legal. The leakage rates (2 or 3 straight truck or 3 or 4 psi combination) in the CDL manual are not in the Regs.

 

Some great responses here, but one thing I have not seen mentioned yet is to check and see if the leak occurs only when the hi/low lever on the shifter is in one position or the other. The gear range shifting mechanism is pneumatic in most trucks.

I chased a leak like what you described for a while. The dash panel valve and another valve that i cant remember off hand were eventually replaced, and the truck hasn't had air issues since.

 

I vote for the fan clutch or line to it. Depending on which style you have.

 

Bet it is the brake valves on the dash, we replaced a bunch of these because of air leaks.

 

To test air system integrity, make and hold a full brake application for
at least 2 minutes while watching the air pressure gauge with the
engine off, the key in the

“on” position, and the park brake off. Leakage, if present, should be less than 5 psi for the 2
minutes. Continue the test by applying the park brake and then “pumping” the brake pedal to
release air. A visual warning should activate when the air pressure gauge drops below 60 psi.

 

Didn’t state there aren’t methods of testing leakage rate, I stated they aren’t in the Regs. If they are what’s the number. Oh and btw, the low pressure warning is required by the Regs to activate before pressure drops below 55 psi, not 60 as commonly believed. 393.51.

 

In the old days of the air starter trucks,you couldn't allow any air leaks at all,or else you'd be looking for a glad hand blow job,from another truck to get you started.If you're leaking air at the rate you described,it might be acceptable for some,but not me.I've had trucks that would hold air for weeks,when shut down.Like the guys say,get the soapy water,and someone with good hearing and get them fixed.It is working your compressor/unloader valve overtime.

 

For those unfamiliar with air start trucks...