air brakes use too much air

What about the relay valves for your drive brakes and steer brakes? Why not put a air gauge on the chambers and see how much air is coming out? There should be a test port.

 

First thing that comes to my mind is you may have a brake booster gone bad or a pancake busted. A brake booster gone bad has to be replaced, but if just a pancake has busted that can be replaced. A serviceable pancake is typically the kind of booster used on steering axles or non parking brake drive axles. I read where you did not hear any leaks, but all I have said only applies if air is leaking WHEN the brake pedal is being depressed down. I didnt know if you had anyone walk around and listen while you hold the brake pedal down. If not, get someone to walk around and listen while you hold the pedal down. I hope this helps and good luck.

 

As FlyingTrucker said, you could have a biscuit leaking only when the brakes are applied. This is especially true of the single chamber cans that are on your steering axle. And, you just said that you had not replaced those.
Check them out.

 

See if your air tanks are full of water. The water will reduce the air capacity of the tanks and result in bigger than normal pressure drops with regular brake applications.

 

How long does it take your air compressor to build air when you start it up?


you've got a problem there somewhere, it shouldn't lose that much air without a leak.