MC8 Air Leak

[NCRealAuctions]

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?

[JackConrad]

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

[ghanson]

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. 

ghanson

[ChuckMC8]

 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-