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2006 Granite parking brakes won't set


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I drive a 2006 Mack Granite cement mixer. Sometimes if I'm holding down the brake pedal too hard ( I live in the Ozark mts, everything is steep) and I go to set my parking brake, the brakes won't set. I don't hear the air release you get when you set the brakes. If I let up a bit on my foot brakes the parking brakes will then set. But if I'm on the steep hill that's hard to do, usually just hold it using low gear then set the brakes. I've replaced R14 valve relay, there is another type valve inline after but before the smaller individual replays. any ideas?

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Maxi brake Chambers have 2 sides, each side can act on the brake rod. If you are flooring the brakes and you pop the park knob, you're trying to use both sides of the brake chamber to hold the truck. You may not hear a hissy from the park brake engaging because of this. 

Have you had situations when trying to set the brakes where they didn't actually set and the truck started rolling?

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On 8/20/2019 at 7:24 PM, JoeH said:

Maxi brake Chambers have 2 sides, each side can act on the brake rod. If you are flooring the brakes and you pop the park knob, you're trying to use both sides of the brake chamber to hold the truck. You may not hear a hissy from the park brake engaging because of this. 

Have you had situations when trying to set the brakes where they didn't actually set and the truck started rolling?

Having a similar issue on ch713 '06 . Having parking brake set pressing brake allows spring to release. Also during start up when building air pressure brakes will release until 80psi

 

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When the yellow button is pressed in, it puts air into the spring brake side of the brake chamber and releases the auxiliary brake. When the brake pedal is pushed in, it supplies air to the service side of the spring brake chamber and applies the service brake.

 

I'm thinking the ch likely has either and sr7 spring brake inversion valve (bendix) or the ir2 (Wabco). (Unless you have a bridge formula semi, which only had a spring brake qrv)

Ron what is yours doing wrong?

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