92 mci 102 c , no air at rear air drain valve ?

[gene burge]

everything seems to be working correctly , but sometimes when i open the drain valve i get air other times , nothing ??

[lostagain]

If you shut off the engine with the compressor pumping, there will be air pressure in that line.

If you shut it off with the compressor not pumping, there will be no pressure.

JC

[DoubleEagle]

Or, if it is an air tank drain, it might be clogged with rusty debris. If that is the case, poke a thin wire up through the valve. Once there is an opening, the air pressure (if it is there) will blast out the remaining debris.

[bevans6]

That is the ping tank, per MCI parlance (I have the original driver's handbook for my bus) aka the air discharge muffler.  It is the first line of defense against moisture and oil in the air system, and it has a drain valve on the bottom.  I added a 1/4" air line fitting so I can connect my compressor there to air up the bus.  It's in the main line from the compressor to the air dryer to the wet tank, and when the governor unloads the compressor and signals the air dryer to purge, all of the contents of the air dryer and that line are blown out the purge valve.  It remains empty until the governor switches the compressor on again, and closes the purge valve on the air dryer.  So JC is correct, if you stop the engine while the compressor is charging, everything is closed and there is pressure in that line.  If you stop the engine while the compressor is not charging, everything is open and there is no air in that line.  This brings up a point of trivia for those who drive in freezing weather - it's good practice to fan the brakes down and get the air compressor in a charging mode before shutting down for the night.  That way, if the purge valve freezes it freezes closed, and you can start to build air in the morning when you start up again.  By the time you are ready to go, your valve heater will have melted the frozen valve and you'll never know.

Brian

[buswarrior]

And if the air drier purge valve is frozen in the purge mode, you will get one ton of air out the discharge muffler drain, as the compressor is pumping to its physical limitations against a closed purge valve, and no governor sensing from the other side to tell it to stop.

Winter is coming... is your bus air system running dry today?

happy coaching!
buswarrior