Air leak- any help?

Check the air valve that operates your air bags on the back of the truck, that can go bad and dumps air pretty quick, also check all air tanks valves that you drain the tanks with sometimes someone will open those a crack being a jerk. other wise check all fittings and air brake cans.

 

PB, both of those could, and most likely would, leak with the truck off as well.

Does the leak stop with the air compressor engaged, or disengaged, with the engine running?

 

You say it only leaks with the engine running; however, I must as if the leak can be observed with the ignition on/engine off? If so, I would look at the engine fan clutch, solenoid, & plumbing as a possible cause. Could also have something to do with the A/C system--some trucks use electric-over-air controls there also. Can't think of anything else on the secondary side--other than steer axle ABS valves--that would have any connection with the "key switch". What make, model, and year of truck?

 

check your spitter on your air dryer. start your truck up let it run till your dryer pops off then listen or reach under it and feel if its still leaking.

Another way to check is to just bypass the air dryer altogether.

 

If its only when the engine is started it must be fan related, bend the hose to the fan hub and see

 

If you have a manual switch for the fan just turn it on. You'll hear it if its leaking. If not it should only leak when the fan is running so you wouldn't notice the air loss all the time.