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Author Topic: Air leak  (Read 849 times)
Derrick Thomas
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I may be nuts, but only for buses


« on: October 16, 2013, 01:45:39 PM »

I know this has been visited many times before but here it is. On my mc8 (with dd3) I have a pretty good hiss of air coming from under the floor in the spare tire compartment. I believe it to be associated with the brake system because when I set the park brake it stops. Release the park brake and the air flows. Making a service brake application has no effect. Brake diaphragm or inversion valve? What say all you bussin experts?

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There are three kinds of people in this world....those that make things happen, those that watch things happen, and those that just wonder what the heck is happening. Which one are you?

1977 MCI Crusader MC-8
95% converted (they're never really done, are they?)
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"The Mighty GMC"

« Reply #1 on: October 16, 2013, 03:02:56 PM »


Sounds like a brake diaphragm leaking.

I have had this exact failure before.

If you have a large hose clamp (special vise grip), you can pinch off each side, one at a time, to find the offender.

Good luck in your hunt,


1975 GMC  P8M4905A-1160    North Central Florida

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1979 MCI MC5Cs 6V-71 644MT Allison, Roseville, CA

« Reply #2 on: October 16, 2013, 06:10:43 PM »

If after that does not do it try looking in the spare tire compartment or the tool compartment with someone pushing the push pull parking brake down and it starts leaking with a spray bottle of soap and water, spray the bottom of the peddle brake valve. If that isn't leaking spray your shuttle valve.


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'68 MCI-7 892T, 740 Richard & Missy - Texas

« Reply #3 on: October 16, 2013, 07:28:34 PM »

Thomas, I just replaced my inversion valve that was doing what you said . Got it at napa, be sure and take it with you and mark your connections with finger nail polish or paint.

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