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Author Topic: 4104 brake part question  (Read 570 times)
John Z
1959 GM PD-4104 4139 Northern Minnesota
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« on: August 13, 2013, 02:58:17 PM »

I am trying to chase down some air leaks. My bus has 30-30 piggyback cans in back. I need a new relay valve back there, as it is dumping a lot of air. I can't say for sure the relay on the bus is an original or not. The Mohawk site came up empty on the original part number. What relay valve have any of you used with the 30-30's?
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« Reply #1 on: August 13, 2013, 03:42:55 PM »

You can bet it is not original with the spring brakes,about any relay will work the most common is the R-12 the R-14 is a good relay it has the quick release and anti compound feature built in if you have those separate any relay will work if yours is round it is probably a R-6   
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John Z
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« Reply #2 on: August 15, 2013, 07:00:10 AM »

Thanks Cliff. I guess I hadn't thought it threw when I mentioned an original relay. That would be a couple holes short. It was the end of a long day. A local guy thought a R8 would work. Do you know what difference there is between R8 and R12? I am leaning towards your suggestion. Also I am not clear on the difference in the R14. Thanks again.
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« Reply #3 on: August 15, 2013, 08:06:25 AM »

Isn't this a case where a leaking diaphragm in the brake chamber could cause it to seem like the valve is leaking?  Maybe it is another brake valve I am thinking of.
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John Z
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« Reply #4 on: August 16, 2013, 07:31:59 AM »

Hey Brian. That was my first guess too. I think we have eliminated that possibility by airing up the bus, shutting off the engine, and releasing the brakes. There is no change at all in the sound of the air moving back there. It is identical whether the park brakes are on or off. Can actually feel air at the relay.
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Cary and Don
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« Reply #5 on: August 16, 2013, 08:15:56 AM »

I would still check the lines going to the chambers and see if that's were it's coming from.  We chased one of those leaks, rebuilt that  valve twice and it wasn't the valve at all.  Air was leaking at the park brake valve to the park chamber and then to the relay.  That valve is a lot easier to fix than the relay.  We rebuilt that relay valve without taking it out. 

Don and Cary
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