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Author Topic: Air Problems v3  (Read 499 times)
Jnbroadbent
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« on: May 18, 2013, 06:48:17 PM »

Rebuilt air purge valve, fired it up (after figuring out how to prime it) and it fills up pretty fast until 80-85psi. Fast idle and skinner don't even activate.  Are there any check or release valves that open up around 80-85? Emergency brake won't release either.

Man it's like a bunch of gremlins decided to come out at once.
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Jon
1980 Mc9 w/ veg oil
8v71
Jacksonville Fl
bevans6
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1980 MCI MC-5C




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« Reply #1 on: May 19, 2013, 04:08:11 AM »

Your generation of bus is like mine, the flow is from air dryer to wet tank to dry tank to E-filter to pressure protection valve to the rest of the system.  There is one check valve on the output of the air dryer, and one on the input to the dry tank.  Pressure gauge reads from the dry tank.  Accessory system, including the engine stop skinner valve and the fast idle skinner valve, are after the pressure protection valve, as is the parking brake tank.  The inversion valve that controls the parking brake lock action at the DD3 cans is basically fed from the parking brake tank but the pressure that is used to unlock them after the button is pressed comes from the dry tank acting on the main service brake diaphragms so if the pressure is only 85 psi, it's won't be enough to unlock the brakes.

When this happened to me it was a big leak in the accessory system dumping air out.  The pressure protection valve is designed to open at around 65 psi, so under 65 psi it is closed to retain air in the dry tank, above 65 psi pressure is passed on to the accessory system so that's when the leak stops pressure from building.

Brian
« Last Edit: May 19, 2013, 04:12:44 AM by bevans6 » Logged

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
Jnbroadbent
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« Reply #2 on: May 19, 2013, 10:20:14 AM »

Your generation of bus is like mine, the flow is from air dryer to wet tank to dry tank to E-filter to pressure protection valve to the rest of the system.  There is one check valve on the output of the air dryer, and one on the input to the dry tank.  Pressure gauge reads from the dry tank.  Accessory system, including the engine stop skinner valve and the fast idle skinner valve, are after the pressure protection valve, as is the parking brake tank.  The inversion valve that controls the parking brake lock action at the DD3 cans is basically fed from the parking brake tank but the pressure that is used to unlock them after the button is pressed comes from the dry tank acting on the main service brake diaphragms so if the pressure is only 85 psi, it's won't be enough to unlock the brakes.

When this happened to me it was a big leak in the accessory system dumping air out.  The pressure protection valve is designed to open at around 65 psi, so under 65 psi it is closed to retain air in the dry tank, above 65 psi pressure is passed on to the accessory system so that's when the leak stops pressure from building.

Brian

So because it's airing up past 65 I can safely assume the pressure protection valve is not the culprit? (Though I can't rule it out until I get my hands and eyes on it)

Come to find the accessory tank (underneath driver) valve was open so I was hopeful it was the leak, closed it and still the same thing. I'll make some wooden ramps so I can stop digging a small trench to get under the bus (blocked and cribbed of course)

I can kind of hear what sounds like a tank changing air pressure (not a hiss) Hopefully it's just a drain valve
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Jon
1980 Mc9 w/ veg oil
8v71
Jacksonville Fl
robertglines1
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« Reply #3 on: May 19, 2013, 03:36:26 PM »

Air bags inflate?
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Bob@Judy  98 XLE prevost with 3 slides --Home done---last one! SW INdiana
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