Help tracking down air leak

If you have a bad maxi chamber, it will most often bleed off through the quick release valve exhaust on bottom of valve, although I have also seen them bleed off through the foot valve exhaust port under hood on firewall.

 

Also check the brake pedal and foot valve plunger for dirt build up, corrosion, hanging up etc. This is a very common intermittent problem. Should be free of dirt and work freely.

 

The maxi is the dual chamber combination of parking and service brake. Keep in mind as you're working through this that just slight downward pressure on the footvalve plunger can cause this problem and only do it at times like you are experiencing.

 

The valve can be hanging up even with a free plunger and pedal. If you can get it to act up you could be more sure. If you can't get it to act up, all you can do is test the chambers to eliminate them as the cause. I am swaying towards the footvalve being the problem. Does your truck have park brakes on both drive axles?

 

To answer your question why is foot valve related. The foot valve is supplied with air from both sides of the air system but kept separate by way of the spool.

 

On a Pete there are air relays behind the panel behind the brake pedal. They are mounted in a manifold thats bolted to the floor. As the relays get weak thay will start leaking. Had the brake light relay go bad on mine is how I know. Haven't had the kenworth apart down there since I haven't had a issue but I would think they are similiar.