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.

 

Whats a maxi chamber?

I had my foot brake valve freed up last month. It was corroded and now its very free and works perfectly. They lubed up the valve and it actually took me a bit of getting used to since the brakes now are more touchy!

 

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.

 

Why is the foot valve related (out of curiosity)...

Ok..Well the foot valve is perfectly 'free' and not hanging up or sticking (as far as I can tell) and has good travel and motion.
Before I had them fix it, it wouldnt always release. The pivot pin was corroded. They took it all apart and cleaned things. Works great now...

I had the trouble before and after the foot brake valve repair.
Hmm..

 

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?

 

No. Only spring brakes on the REAR REAR. The Front REAR are single chamber/single air lines only.

I pinched off the air (using Vise Grips) to the spring air-hoses on each 'maxi' chamber (One at a time and then both at the same time) and while the parking brakes DIDNT release (as expected) I was still hearing the air hiss/flow inside the dash...so I dont "think" the chambers are leaking.

I know I have ABS on this truck, but have NO idea if it works or not...Not a clue how to check/test it either w/o slamming on the brakes.

 

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.

 

I have a floor mounted brake pedal. The only thing I see is the valve, when I look under the floor from within the engine compartment...