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Author Topic: MC8 Air Leak  (Read 2939 times)
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Posts: 10

1975 MC8

« on: May 04, 2006, 08:43:33 PM »

My 75 MC-8 is significantly leaking air out of the rear main brake valve relay exhaust port. It takes almost 20 minutes tp pump the air pressure up to 90 lbs. I replaced the relay valve with a new valve, and the new one leaks the same way out the exhaust port. I've noticed when airing the bus up that it will air to approx. 30 lbs. & stop & if I hold the parking break down the air will go on up to 120 lbs and then blow off excess air. When I cut the engine off with the parking break locked the air leaks out of the valve relay exhaust port to 0 lbs in less than 5 minutes. I discovered that if I release the parking brake the air stops leaking rapidly. This is not good. Do any of you Busnuts know how I can fix this very frustrating problem?
Orange Blossom Special II
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73' MC-8 8V71/HT740 Southwest Florida

« Reply #1 on: May 05, 2006, 03:42:25 AM »

Sounds like it may be a leaking diaphram in one of the rear drive axle brake cans. These have 2 diaphrams in them (1 for normal braking and 1 for parking brake).  The parking brake hoses to the brake cans are the ones nearest the front of the bus (rear of brake can). Clamp each hose (one at a time) with a pair of pliers and see if the air leak stops. If this corrects the problem, you need to replace that diaphram.  Hope this helps, Jack

Growing Older Is Mandatory, Growing Up Is Optional
Arcadia, Florida, When we are home
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MC 9

« Reply #2 on: May 05, 2006, 01:46:00 PM »

I agree it sounds like a leaking brake diaphram.  I had the same problem this spring.  Not too difficult to fix, but let me tell you, they're not giving the brake diaphrams away.  A set of four, if I recall was about $300, that's one for the main brake and one for the parking brake. 


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1977 MC8 and 1993 102C3 Temple Ga #322 F&AM

« Reply #3 on: May 05, 2006, 02:23:41 PM »

 The DD3 Diaphragms from Mowhawk 1-800-323-7652 are
Service Diaphragm is 4C-5-20 are $63.05 each
Auxillary Diaphragm is 4C-5-21 are $16.78 each

I paid $159.67 for all four, and they didnt charge any additional for shipment.
hope this helps-

Far better is it to dare mighty things,to win glorious triumphs,even though they may be checkered by failure, than to rank with those poor spirits who neither enjoy nor suffer much,because they live in the gray twilight that knows neither victory nor defeat.  Theodore Roosevelt (1858-1919)
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