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Author Topic: air leak  (Read 931 times)
dwerkheiser
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« on: March 15, 2014, 01:02:22 PM »

My 78 MCI MC-8 has an air leak. The air is leaking from the bottom of the E-10 valve. As soon as I put slight pressure on the brake rod the leak stops. Where do I start looking for a cause of the leak or is the E-10 going bad.  Don't want to start throwing parts at it, would rather ask for diagnostic help. 
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1979 MC8, 8v71, 730 allison
bevans6
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« Reply #1 on: March 16, 2014, 06:05:18 AM »

That is the exhaust port of the E-10 valve.  It's purpose is to allow residual air to escape from the supply lines to the front brakes and the relay valve after the brake pedal is released.  On your bus, the primary circuit applies both front and rear brakes, while the secondary circuit supplies the emergency brake through the secondary ports after the shuttle valve has shuttled.  In any case, as soon as you apply the pedal a little bit the exhaust valve is isolated from the supply lines and pressure is supplied to the brakes.  This suggests that the E-10 valve itself is not the issue, but that's by no means certain, just a good sign that the E-10 is doing it's thing.  The most likely cause of that air leaking is something in the rear brake system, most probably the inversion valve or one of the DD3 brake canisters.  You can diagnose by pinching off or removing brake lines to see which one has air when it shouldn't have.  If the leak occurs when the parking brake is on, suspect a leak in the parking brake diaphragm.  If the leak occurs with the parking brake off but no service brakes applies, it could be the shaft lock on the DD3 - it has air pressure whenever the parking brake is not applied.  If you google Bendix DD3 and E-10 you will find their PDF manuals for both devices.

The DD-3 brake cannister basically has three modes in can be in.  First is normal operation.  There is pressure at the locking port to release the pushrod, no pressure on the parking brake port, and variable pressure from zero to 120 psi on the service brake port.  Second is parked with parking brake on ( and normal system pressure in the bus).  In this mode there is zero pressure on the locking port so the clutches engage and lock the shaft out, there is 85 psi of pressure at the parking brake port to engage the parking brake, and there is zero pressure at the service brake port.  Third is emergency brake operation.  Here there is pressure at the locking port to keep the push rod released, there is low or zero pressure at the service brake port (the shuttle valve operates with a pressure differential of 40 psi between the emergency and the service brake supply pressure) and there is variable pressure at the parking brake port ( which is now really the emergency brake port) as the E-10 valve secondary system supplies braking pressure via that port.  

When the brakes are released in normal operation, there is a quick release valve in the front brake system and in the R-8 valve at the rear brakes to release air.  The exhaust valve releases any residual air in the front brake system and the relay line from the E-10 to the R-8.

Hopefully this gives you some hints as to how to troubleshoot your system.

Brian

« Last Edit: March 16, 2014, 06:11:02 AM by bevans6 » Logged

1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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Vintage race cars -
1978 Lola T440 Formula Ford
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Nineforever
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« Reply #2 on: March 16, 2014, 08:10:16 AM »

I had a simmiliar problem with my 102 built air from start perfectly like it should , then when i released the maxi brakes it would have a hard time keeping air pressure up unless i had my foot into it , ran it up on the ramps so i could listen for air leaks could hear nothing ... changed out the air dryer still same problem .... then the DD3 brake pots then problem was resolved ... they do have rebuilt pots available also kits if you want to rebuild your self .
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Hyway 3 100 klms south of Yellowknife NWT Canada
dwerkheiser
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« Reply #3 on: March 16, 2014, 08:30:51 AM »

Ok guys, I have done a little more investigation. I also have a leak, that sounds like it coming from the R-8 valve on the rear axle bulk head. It does not stop when the brakes are applied. Also the leaks stop when the bus air leaks down to 60 psi. The parking brake is on and with it off doesn't change the leaking. Does this point to cause like brakes.
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1979 MC8, 8v71, 730 allison
bevans6
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« Reply #4 on: March 16, 2014, 09:07:05 AM »

If the bus leaks down to 60 and then stalls out that points to the accessory system leaking down till the pressure protection valve activates.   Since the problem seems to center around the R8 valve, I might try disconnecting or pinching the main service lines to the brake cannisters from the R8 valve and see if one of them is leaking. 
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
Vintage race cars -
1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
dwerkheiser
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« Reply #5 on: April 20, 2014, 10:32:59 AM »

Ok, guys. You were right on the money with leaking DD3 chambers. Finally got the diaphragms changed out, no more leaks at the tredle valve. Now I have found some leaks at the air cylinder and regulator for the fan tension and also the push pull brake valve has a small leak. Will work on these in the coming weeks.   
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1979 MC8, 8v71, 730 allison
Ed Hackenbruch
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« Reply #6 on: April 20, 2014, 04:18:49 PM »

Ain't it fun?  you hear an air leak and fix it and find that you have another one that wasn't as loud as the first one so you never knew it was there........and when you fix it you find another one that you never heard either. Seems like one leak turns into 4-5 before you are done. Wink
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1968 MCI 5A with 8V71 and Allison MT644 transmission.  Western USA
dwerkheiser
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« Reply #7 on: April 25, 2014, 05:34:00 AM »

Ok, I was wrong again. I still have air exhausting out of the bottom of the treadle valve. The air leak has stopped at the relay valve at the rear after I replaced the DD3 diaphragms. Any ideas on this leak.
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1979 MC8, 8v71, 730 allison
luvrbus
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« Reply #8 on: April 25, 2014, 05:39:17 AM »

Try cleaning under the pedal and be sure it is coming all the way back up a little WD 40 helps, then check the shuttle valve, if you still have the leak it's time to replace the foot valve is that a E10 or a E6 foot valve ?

good luck
« Last Edit: April 25, 2014, 05:47:58 AM by luvrbus » Logged

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dwerkheiser
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« Reply #9 on: April 25, 2014, 05:41:37 AM »

Can the treadle valve be rebuilt.
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1979 MC8, 8v71, 730 allison
luvrbus
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« Reply #10 on: April 25, 2014, 05:49:52 AM »

They are rebuild able if you buy one that is probably what you will find

good luck
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krank
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« Reply #11 on: April 25, 2014, 08:06:04 AM »

If you still cannot find the air leak, try removing the service (application) hoses off the spring brake chambers on the drive axle. Then with the wheels firmly and securely chocked to prevent the vehicle from rolling, release the parking brake. Check for any air leakage out of the service port on the brake chambers. Sometimes the pushrod seals between the chambers and air will transfer from the spring chamber into the service chamber. It will then feed backwards down the line until it finds an exhaust port. When you make a light application on your brake pedal it overcomes the transfer leak and stops any air from exiting the exhaust port of a valve.
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Jim Eh.
1996 MC12
6V92TA / HT741D
(Sunchaser)
Winnipeg, MB.
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