Im new to the bus world as I just purchased my first. A 1974 MC-8.
I love the style and look of the bus.... But now the bad...
When I push the park brake button down i have a large air leak from the QR1 valve in the spare tire compartment. I come from a background of working on tractor trailers which i understand doenst pertain to the busses. but i know that air leaking from a qr valve normally means something down the line is messed up. I thought about a brake biscuit leaking so i removed the service lines from both pods and had no leaking from the lines but still a leak from the QR1.
Ive been hunting for the inversion valve but i dont have anything that looks like the valves Ive seen. I think it is the valve directly above the differential with both park and release lines coming from it... Im guessing that is where my problem is.
I was told that someone looking at the bus before me was pumping the brake pedal while the park brakes were still locked.... which is why i was thinking blown biscuit. But I am at a loss.... Any help would be greatly appreciated.
Eric
Tulsa
That exactly happened on my MC-9. Two things were leaking: 1. Parking brake push-pull valve ; 2. Inversion valve. In my bus, this is attached to the top of the differential and had about 8 fat brake lines connected to it. I replaced all the seals in it, the leaks were stopped. A leaking diaphragm in one of the dd-3 brake actuators could also cause that leak.
????. You say the QR1 valve that is in the spare tire compartment? So that is connected to the service lines to the steer axle, right?
If that's all the bad, that bus is in very good shape LOL!
I diagnosed my problem by hooking up an external air compressor, and then disconnecting the air lines to the brake canisters one at a time and noting when the leak out the front valve stopped.
I'm pretty sure the driver can't break anything in the brake system, by pumping the brakes or anything else. But I just started driving my first bus too, and I could be wrong.
Actually my problem was a leak when my push pull valve was up, but I think you are on the right track with the inversion valve.
Quote from: Jim Eh. on August 28, 2020, 08:50:14 PM
????. You say the QR1 valve that is in the spare tire compartment? So that is connected to the service lines to the steer axle, right?
The front axle doesn't have any dealings with the park brakes. And the QR1 has a copper line that goes all the way to the back.
I was thinking about this a bit more... I answered hastily earlier, without thinking it through completely.
It sounds like you have the air diagram. That's very good, and with a little thinking and testing should lead you directly to the problem.
If there is no pressure on the input side of the QR valve and air is coming out of the exhaust port, there is definitely a leak farther down the line between the brake circuits, brake diaphragms and inversion valve would be the first places to look.
If there is pressure on the input side of the QR valve and air is coming out of the exhaust port, the diaphragm inside the QR valve itself is bad.
If I remember correctly there is no pressure at the QR valve in the Park position and pressure at that valve when the parking brakes are released.
So if this is correct, you should examine the QR valve itself.
My inversion valve looks like this:
The QR1 had a hole in it. After checking the inversion valve and unlocking the brakes with truck air. I appreciate all the advice and insight. I am learning a lot about things I didn't know I wanted to know.
The bus is now home and I can start working.
Congrats and welcome. We really are a friendly and helpful crew to those that accept the responsibilities of a busnut. You've clearly earned your first stripe and are welcome here anytime.