Intermittent air pressure drop at red light.

When losing air pressure, do you hear the leak?
Same problem if you disconnect trailer?
My guess? Brake chamber pancake seal.

 

I'm going to go with a relay valve. ? It sounds like it is sending air out the exhaust port.

 

With or without a trailer?

Standard air brake diagram below.

Anything on the service side, primary or secondary can be leaking.

Treadle valves, relays valves, tractor protection valve can all leak intermittently as the exhaust port will seal one time, then not the next.

Brake chambers can also leak intermittently, such as the center seal between spring brake and service side.

Also, it may just be a coincidence that you are stopped when this happens. Could also be the purge valve at the bottom of the dryer is sticking open and the outlet check valve is also stuck open to allow air to back flow.

One way or the other, from the amount you are losing, it should be easy to find once it rears its ugly head!

 

I am going with xsetra: The regular driver complained about rapid air loss and brakes locking down for several weeks. Local truck that only went to land fill and had been checked at the shop many times. I left the shop with it after a full inspection and service. About the time I shifted to high range air pressure was dropping rapidly and parking brakes were applying. I jumped out and found a chamber dumping air, then the brakes set. Got in the truck, and air built back up, went to the shop and no leaks.
Never saw or heard of anything else that could lose that much air that fast and then work OK for 3 days.
Service brake, parking brake, truck, trailer and how do you test that? Do not get rolled over. One I was driving had no trailer. I turned it off and left it in gear.

 

A piece of debris like a small pebble in a QR valve could cause this.
Hit the brakes the pebble or whatever moves and holds the valve open.
Got the t-shirt for that one.

 

I would look at brake chambers both truck and trailer and air bags too. I chase air leaks tiny ones true all the time drives me nuts

 

Glad you found it. Thank you for the update on the repair. Updating when the fix is found, really helps the forum.