2004 Mack granite strange air problem.

[wrenching4life]

Hey all, im working on a 2004 Mack granite roll off dumpster truck, tri-axle. The issue im having which iv still yet to figure out is every time the service brake is applied their is an instant sneeze of air from one or a pair of valves on the passenger side frame rail, at the rear of the truck. Then the hiss stops and the brakes apply. I can get only about 3 service brake hits with the engine off, and air built to the gov cut out.(aprox 120). Im used to internationals/ pete's and have never run into this before, this does not seem normal to me. Any thoughts on this one? im thinking the sneeze/hiss is not normal when first applying and i should get more than 3 hits on the service brake before the maxi's come on? thanks all!

[j_martell]

The hiss sounds like a leak. I can get more than three hits before the alarm goes off on my 2011 granite tri axle roll off. How much pressure does it loose during the sneeze. Not an expert by any means, but if it's hissing it's leaking.

[troy.maillet.9]

Could be a Trudder valve that splits off to the chambers near the rear by the trailer wiring ?? Or your Diaphragm is cracked in one of your chambers

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[Macktech689]

The most likely cause of your problem is the "combo" valve. It is mounted on the crossmember in front of the drive axles between the ABS modulators. If the hiss of air is coming out of the springbrake relay exhaust port (mounted on the same crossmember, opposite side on the right side) the "combo" valve needs to be replaced.

[rsb502]

Yeah sounds like an combo, that or depending on your air system set up a crusty quick release valve that's slow to close.

[wrenching4life]

yea im still having this issue the valve that is exhausting when the parking brake off and service brakes applied is the valve in the rear that connects to all the emergency side of the brake cans. I still can not make any sense of it. One thing that I do know is with the parking brake off and you apply the trolly brake there is no air exhausted from the same valve... the valve will only exhaust with the service brake (pedal application) thoughts?

[rsb502]

ok the valve that is leaking air is the spring brake quick release valve? this would be the valve that has air lines going to all of the spring brake chambers to the emergency side, is that correct? if so, you may jut have a bad brake chamber andn your service side is bleeding into the parking side, try removing the service lines from your chambers and release the parking brakes, with the parking brake valve pushed in and no service application made there shouldnt be any air coming out of the chambers where you removed the service lines, mif there is that brake chamber is bad.