Bendix AD-IP Air Dryer question..

At 1300 rpm and higher you have manifold pressure which is where the air compressor intake air is supplied from. The manifold pressure in effect force feeds the air compressor with intake air making it operate at higher efficiency, something the compressor can't do when idling with no pressure in the intake manifold.

 

I had same problem. truck sneezing like it had a bad cold. unloader was bad & had good size leak at fan clutch, but leak at clutch would not show unless truck was running. soaped it good, let it run 5 min saw not much at all, but a few min later the suds would fill up all over. very many tiny leaks. if pressure stays high 110-120 most all the time & purges very frequent, it is a sign of a leak. yeah it could be governor, i replaced mine, to no avail. could be dryer, i replaced mine to no avail. mine was the result of leak at clutch & unloader. if pressure stays high it is because the compressor is constantly running trying to keep up with lost air, resulting in air press staying in the 110-120 & purging too often.

 

by the way. bought spare unloader kit for $12.00 kep in my side pocket.

 

AD-SP use the air tank for purge volume, so the pressure drop is normal as the Bendix info says. I think you're tying two things that aren't related together: the pressure drop you're seeing isn't a symptom of the problem, which is a leak somewhere prior to the tanks. I'm still betting something is up with the unloader because raising the rpm solves problem; a sticking purge valve causes a different problem in that it blows compressor air out the vent until it seats, when the compressor first starts, usually you can hear the 'ptt-ptt-ptt' as the compressor strokes from the area of the dryer. Get an unloader kit, Bendix manuals are all online, it's an easy thing to do. Much less hassle than disassembling the dryer.

If the unloader doesn't fix the issue, then pull the dryer and bench test it.

 

A few bolts on top the compressor hold the unloader plate in place, pull the old one out, clean up any gunk in the bore, pop the new lubed one in.
http://www.mecaniqueindustrielle.com/service_data_bendix_compresseur_tu-flo_550_pistons_anglais.pdf

 

on my truflow 550 it is 2 7/16 bolts take off top cover plate replace parts, springs, gasket, couple metal things. you'll see when you take the bolts out, replace everything you take out with whats in the kit. you will see how easy once you do it.