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No air from wet tank drain
Wet tank draining in rear passengers engine access has no air coming out of it when valve is opened. Crawled under the bus and checked wet tank, has air in it (no water).
Any idea’s why there is air in the tank but not coming out of rear wet tank drain?
I did try filling from the Schrader valve and air went in without any issue, but no air out when valve opened.
Chuck
2000 Featherlite H3-45
2024 Jeep Grand Cherokee
Leesburg, FL -
12 Replies
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Bad news.
It is highly probable the pneumatic system has or had a serious moisture problem. Failure to maintain the air dryer, or failure to drain the tanks at every service typically results in a wet oily sludge accumulating in the wet tank and that combination of water from the compressed air, oil from the compressor, and rust eventually reaches and blocks the drain.
Good news.
If you are lucky no water progressed beyond the wet tank.
Really Bad News.
If water went past the wet tank and got into the primary, secondary and accessory tanks there is a high probability the function of various valves has been compromised because that moisture really messes up protection valves, Norgren valves, etc.
If you open the valves on all the tanks in sequence, starting with the wet tank (take a wire or thin rod to poke a hole in the crud blocking the air flow out of the tank) each succeeding tank should have air only in them. If they also drain water it is a safe assumption you might also have air in accessories like air bags, flor slide, etc.
Several members have experienced what I described above, and it is a serious problem that must be corrected. Tom and I have preached about the need to routinely drain air tanks and service the air dryer to keep the system dry, but also to verify the check and protection valves are functioning as required because their failure is a serious safety concern.
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Disregard.
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Huh????????
Why would anyone put a check valve in a drain. You don’t want air to enter through a drain, you want what is upstream from the drain to come out. A check valve defeats the entire purpose of a drain.
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I never cease to be amazed at what people do to their coaches to fix a problem instead of doing it right.
The original equipment tank drain petcocks are reliable, available, cheap and yet if one leaks why not just put a pipe plug in the drain until a new valve can be put in place?
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We agree 100%.
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Ah you boy’s are confusing me… That shrader valve is used to charge the wet and brake tanks with air. Look at the sticker, air brake system fill.
Jeff and Lindsay
91 Marathon XL -
Jon:
Agree with everything you said BUT
When I purchased the bus about 1 1/2 yrs ago the first thing that was done was to have the air dryer serviced. The old cartridge was in good shape and the tanks did not have water/oil in them. I have religiously drained the front aux tank and back wet tank during use of the bus. At no time has any water or debris come from the wet tank drain in the back, I have had water only in the Aux tank drain in the front (I am assuming caused from the aux compressor). When the rear wet tank drain stop working recently I crawled under the bus and drained the wet tank from the tank valve a couple of times. Each time air (plenty of it) came out of the tank but never any water or oil, so I think I am safe on the issues you addressed. Still have to drain air in sequence though.
Tim:
Air used to come out before so I would quess there is no check valve.
Tom:
Drivers pre check list in the Prevost manual calls for daily draining the aux tank and the wet tank at the end of each day. Manual specifically calls out the drain cocks in the front LH drivers compartment and the rear RH eng compartment. also notes the Schrader valve in the RH eng compartment for filling the tanks if need for towing. As indicated in the yellow circle on your picture.
Pneumatic drawing shows wet tank drain goes to bottom of air dryer which then T’s into the Delivery line from the compressor. Air dryer also has two more lines on it, one to the governor and the other to the wet tank. Pretty staighrt forward.
Definitely confused; Air goes in but not out like it used to. Only clean air comes form wet tank drain. No moisture, oil, sludge, nothing but air. Wondering is there is a valve in the bottom of the dryer that’s sticking? haven’t studied the dryer break down closely yet.
2000 Featherlite H3-45
2024 Jeep Grand Cherokee
Leesburg, FL -
My bad. I was thinking air tank drains.
The valve Tom described will not have air in it except when the compressor is running or just after it has turned off.
That valve is to air up the coach, all systems as Tom describes. The one in the steer compartment is for accessories only.
I believe these senior moments are coming more frequently.
2000 Featherlite H3-45
2024 Jeep Grand Cherokee
Leesburg, FL -
Yep, the only time you’ll get anything coming out the drain while not running is from what air is in the line and sometimes that will have a little moisture.
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The two schrader valve fittings only hold pressure for a while but I never knew them to be appropriate for draining the accessory or wet tanks.
A well maintained pneumatic system with routine dryer element replacement (I do 2 years) is likely to keep moisture from the wet tank and all tanks downstream. But converters depending on how the coach is set up can have moisture enter the accessory (auxiliary) tank from the aux compressor, especially if the converter has not provided a good water trap or dryer.
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Jon & Tom:
Everything you said is true. Air only comes from wet tank drain in RH rear engine compartment when (and only when) compressor is running. Here is where it gets clear as mud.
Per attached page of owners/maintenance manual it is suggested that the wet tank drain in the RH rear engine compartment is purged daily along with draining at the bottom drain every 12,500 miles.
After Jon & Toms post I reread the pneumatic drawings and crawled under the bus again to visually check the hoses. The air hose goes from the schrader valve/wet tank drain in the RH rear eng compartment to a T fitting at the bottom of the air dryer (other side of T fitting is the supply air from the compressor). It is in no way connected to the wet tank, so the owners/maintenance manual is misleading saying this is a wet tank drain. I even called Prevost PASS and they confirmed the plumbing is “As Built” and noted the only purpose for the air line running to the RH rear eng compartment is for FILLING the wet tank which in turn fills the primary, secondary, and Aux tanks. The one note the PASS rep said that by filling through the rear schrader valve you are passing air through the air dryer before it enters the system assuring dry air for the system.
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