Airbrake intermittent issue

The truck is a 2004 Sterling and has developed a mystery brake issue. Every once in awhile when the service brake is applied there is air escaping somewhere on the truck (around or near the axles or frame). If the brake pedal is released and re applied it usually stops, also there isn't a significant pressure drop on the gauge. The last two times this happened the brakes on one or more of the wheels did not release properly, I was however able to move and pull off to the side, so I don't think they were completely locked. Pullin the parking brake and releasing it seems to take care of the problem (dam, I should have just set the trailer brake and crawl my ### under there and see what was locking up... just thought of this)
My mechanic and I were messing with it yesterday but could not duplicate the problem. We found a small leak on the tractor protection valve so we replaced it but the problem came back again today so that wasn't it. As I said it only does this whenever and I can't duplicate it on demand. Any thoughts are appreciated.

 

You might have a point. Like I said pulling and releasing the parking brake takes care of it.

 

Well it took awhile and some parts but I think it's fixed now. We eventually found that the air was coming out through the SR7 valve when the pedal was depressed, basically acting exactly as Bendix demonstrates the Spring Brake Modulation feature in their SR7 valve video. After replacing the valve the problem returned so we replaced the foot valve next and still no success. After all this I started noticing a pattern, the problem usually happened shortly after loading or unloading (after the PTO has ran). The PTO is tied to the brake system and even sets the brakes when engaged. After bypassing this feature the issue had resolved.