Air issue.

That’s takes 2 individuals to check for a major leaks. One in the truck stepping on the foot brake and the other to see where the lose air is going.

 

Stepping on the brakes air leaks.

Likely issues.

Trailer brakes set still leaks, truck problem. ABS valve, ABS Modulator, steer or drive service hoses, brake chamber.

Trailer brakes set doesn't leak, trailer problem. Quick release valve on service air line back of cab, service air line, service gladhand seals, slider lines,
ABS Valve, service air line to valve or brake chamber, brake chamber.

 

Set the trailer brakes only, hit the brake pedal, see if it still drops. If not, Trailers leaking.

 

My tri axle was purging every 20 seconds or so ended up replacing the air dryer filter then it purged when needed.

 

Could be a weak purge valve. Had that happen on the last two dryers I had. Last time it was purging too often, eventually leaking constantly, not holding any air, had to bypass it. Both times the dryers were overdue for replacement. A couple fittings to bypass is handy to have, along with a spare govenor, some rubbing alcohol ( winter time) in case of emergency. Whole AD9 reman from Freightliner Dealer is pretty cheap. Better than trying to rebuild or replace the spitter. Fleetpride is another place that usually has them in stock at a good price. $160 or less. Probably $200 now, Lol. Just check the spitter valve, see if it’s seeping air. Watch out for the purge!! Might want to shut the Truck off first. If it’s seeping, you can usually push it in with a small torx or Phillips screwdriver, and get it to stop. Sometimes brake clean helps. But it’s days are numbered. Otherwise check that it’s purging at the right pressure on the guage. 125 is good. If it’s purging below that, try to adjust the govenor, to make it purge at 125. If it still purges too soon, it’s probably weak, and will soon leak constantly, shutting you down. I’ve gotten a bad Govenor once, Aramak rebuilt. About 1/3 price of new. Not worth it. Pay the $60 for a new Aramak or Bendix. I usually keep a spare. Has come in handy at least 3 times over the years.