What are these tanks and valves?

[wrenchtech]

I am going to be surprised if these are part of the air brake system. These tanks reside along the outside of the frame rails on the driver side of the bus right next to the air conditioning condenser assembly, not too far behind the front wheel. They are down low but concealed by the body panel and so they're easily accessible from beneath the bus. And then on the other side of the bus, accessible through a little door down low on the body, there is this little bank of quarter turn valves with labels that correspond to the labels that are on the tanks indicating the words, Primary Secondary, Accessory, Wet. I will admit that I haven't looked very hard or tried to trace down the lines coming to and from these components.. Instead I am hoping that this will be very familiar to someone else who can lift me out of my state of ignorance with a few sentences posted in this wonderful forum. The bus in question is a 2008 Thomas Saf-t-liner HDX as indicated in my profile.

[somewhereinusa]

Those are air tank names. Valves are probably water drains.

[cadillackid]

yep those are air brake tanks.. read up on any of number of youtube viseos that bendix puts out on Bendix air brakes. and you'll learn what each one is for.. much better than me explainng it here
-Christopher

[Truthseeker4449]

Wet is the incoming air from the air compressor. Primary and Secondary are for the air brakes. IIRC Primary is the drive axle and Secondary is the steer axle but I could be wrong about that. Accessory tank holds the air used by everything else.

The bank of valves is a super nifty thing for draining the tanks. I wish more vehicles had them as you are supposed to drain the tanks of moisture daily as water in an air brake system leads to a number of issues. Buses get neglected frequently because their tank drains generally aren't accessible without crawling under the bus.

[family wagon]

Air from the compressor goes to the wet tank, so named because air coming from a compressor always has water vapor -- it's wet. That water hopefully condenses in the wet tank before the air flows into the primary and secondary tanks.

The primary system probably feeds air to the rear axle brakes and the secondary system feeds air to the front axle brakes. The brake pedal controls the air to both circuits at the same time, keeping them separate so that if something fails in one brake system the other system remains functional.

Finally, (if I remember correctly) the accessory tank would be fed off either the primary or the secondary tank. It uses a protection valve so air flows to the accessory tank only when the feeding tank has pressure above some level. This is so that if the compressor is impaired or fails outright while driving the accessories will be starved for air before the brakes are.

[wrenchtech]

This bus has air ride, front and rear, as well. I imagine that it gets its air from the "accessory" tank.

I hear air blowing off every so often, so I am thinking there must be an air dryer somewhere that drys the air for the brakes at least. Where does that usually fit into the air system?

[Brad_SwiftFur]

If you're getting an occasional discharge of air that lasts half-a-second or so, it's likely an air dryer doing exactly what it's supposed to be doing. Many are set up to "purge" each time the air compressor "cuts-out" (reaches full pressure and stops compressing more air). "Cut-in" is naturally the opposite, the pressure threshold at which the compressor resumes compressing air (typically 90-100 PSI).


Brakes usually draw off the primary and/or secondary tanks. The accessory tank may have a protection valve (and many of the accessories will draw off the "protected" system), basically this protection valve opens/shuts around 60-80 PSI and is designed to maintain a reserve for the brakes in case something breaks and a major air loss occurs (including the air ride suspension).

[TJones]

Originally Posted by wrenchtech

This bus has air ride, front and rear, as well. I imagine that it gets its air from the "accessory" tank.

I hear air blowing off every so often, so I am thinking there must be an air dryer somewhere that drys the air for the brakes at least. Where does that usually fit into the air system?

The drier is plumbed in between the wet tank and the other tanks. Not a bad idea to see when the air dryer element was last changed. Vehicles in regular service should have the air driers changed yearly. If you get moisture to drain out of your primary, secondary or accessory tank then the air drier element needs to be replaced.

Ted

[musigenesis]

Originally Posted by TJones

The drier is plumbed in between the wet tank and the other tanks. Not a bad idea to see when the air dryer element was last changed. Vehicles in regular service should have the air driers changed yearly. If you get moisture to drain out of your primary, secondary or accessory tank then the air drier element needs to be replaced.

Ted

I had my air dryer cartridges replaced when I first got my bus; the mechanic that did it said it was likely that they had never been replaced before (bus is a 2003). I suspect I'm going to need a new air system before long.

[Booyah45828]

Originally Posted by TJones

The drier is plumbed in between the wet tank and the other tanks. Not a bad idea to see when the air dryer element was last changed. Vehicles in regular service should have the air driers changed yearly. If you get moisture to drain out of your primary, secondary or accessory tank then the air drier element needs to be replaced.

Ted

Driers are plumbed between the compressor and the wet tank. Not all buses are equipped with driers.

The cartridges should be changed yearly if used in regular service, however, most buses on here aren't used that frequently so the service interval can be extended.

For instance, I've never serviced mine. The bus isn't used enough, and I drain the tanks before and after using it.

[TJones]

Originally Posted by Booyah45828

Driers are plumbed between the compressor and the wet tank. Not all buses are equipped with driers.

The cartridges should be changed yearly if used in regular service, however, most buses on here aren't used that frequently so the service interval can be extended.

For instance, I've never serviced mine. The bus isn't used enough, and I drain the tanks before and after using it.

Your right air drier before wet tank. Maybe it is only a "wet" tank if there is no air drier. Other diagrams just call it a "supply" tank. I think the purpose of a wet tank if there is no drier is to have a place for the air to cool and condense the water before it gets I to the brake system.

Ted

[Brad_SwiftFur]

Originally Posted by musigenesis

I had my air dryer cartridges replaced when I first got my bus; the mechanic that did it said it was likely that they had never been replaced before (bus is a 2003). I suspect I'm going to need a new air system before long.

It's probably not as bad as you think. Drain the tanks and service the system, replace things as they fail (which should be rarely as air systems are pretty robust).

Originally Posted by TJones

Your right air drier before wet tank. Maybe it is only a "wet" tank if there is no air drier. Other diagrams just call it a "supply" tank. I think the purpose of a wet tank if there is no drier is to have a place for the air to cool and condense the water before it gets I to the brake system.

Ted

Even with an air dryer, it's not a "moisture eliminator", there will still be some water get into the wet tank, just much less of it.

[Bon Voyage]

Open those valves to drain your tanks daily and leave it stored with the valves open to keep moisture out of the system.