Basic air system checks.

[Geoff Weeks]

I am new here, but noticed a few air system questions of late. Some of this will be "old hat" but even some oldtimers might pick up a tip.

Start the truck, and let it build air until the air gov shuts off the flow and the air drier purges. Shut off the truck and look at the two pressure gauges, make a note of their readings.

Go around the truck and listen for air leaks. You shouldn't hear any.

 Trucks built since -121 all have at least three air tanks. Wet, Primary (or tank 1) and secondary or tank 2.  Each should have a drain valve.  The wet tank is the 1st tank after the drier or compressor if no drier is used. (Some driers have little purge tanks, attached to them, I am not referring to theses).

 Go to the wet tank and open the drain, it should be dry or almost dry. Allow it to drain fully of air. Go look at the dash gauges and they should remain aprox where they were. If a tank gauge or both fall, you have a defective check valve and no safety for your brakes, one hose bust and you loose 1/2 or everything. Automatic fail. I should note that some air driers (Like the Rockwell/Meritor spin on) use a little air from one of the trucks tanks to purge, you'll see a small decrease in one tank, but it should not fall below cut in pressure.

 Next go to one of the two other tanks, open the drain, it should be totally dry air escaping. Let 1/2 the pressure escape, then close the tap and go look at the gauges, one will be low (say 50 psi) and the other MUST remain aprox where it was.

 Next go to the remaining tank and open the drain, again it should be dry air. Let it drain all the way, again check gauges, one should show some pressure (say 50 psi) and the other 0, If both show 0 you have a leaky check valve, again Automatic fail

 Re start engine and note low air light, buzzer or wig-wag (Anybody remember those?) When the light and buzzer shut off go to the air drier and put a hand near (but not against) the outlet of a purge. Allow the truck to build until the drier purges and note the time. You should get an initial burst of air followed by a much slower release of air for about a minute (exact amount of time is not important, just that it is not a single quick high pressure burst). After the low pressure air escapes no air should come out. Keep watching you watch and time how long it takes until the system purges again, 10 min minimum!  If it purges before that time, you have work to do.  Less than 5 min and you have a serious leak.

 If you get one quick, high pressure burst, then nothing, you have a drier problem. You will likely also notice moisture in the tanks,

 I don't know how many times I have heard " My system is great, it pumps up to 125 (or whatever) and never moves" Ah, no, that mean you have a problem, likely a leak in the "wet side" that keep the compressor cycling before the primary and secondary res have "used" any air.

 Gauges should rise to shut off pressure, then fall to "cut-in" pressure (useally 20-25 psi below cut-out) then rise again. If yours are staying at cut out and never failing in between purge cycles, The air is leaking from before their check valves. Wet tank, gov, compressor, fan clutch, drier and piping between all these are suspect.

 I have stated in another post, running down the highway (no brake applications) 10 min between cycles in the MINIMUM I would accept. I had one semi combination that would go 45 minutes!

 Most DOT cop's haven't got a clue what they are doing when the do a level 1 air check, and god help you if you find one that does and has a chip on his shoulder! If you can pass the above tests, and a normal pre-trip you should be ok  with the exception of the TP which hasn't been checked in the above.

 This was posted by another member and has a good diagram of the air system basics.

https://www.doverbrakeinc.com/downloads/catelogs/BENDIX AD-9 AIR DRYER.pdf

Edited July 6, 2023 by Geoff Weeks
Add info

[Swishy]

Gr8 info Geoff

our older trucks have drain taps fitted to bottom of air tanks

once a week we let the tanks drain over night and note any oil or sludge on the ground next day

With the compressor govonor cut out @ max 120

the commpressor is free wheeling and  the air is sukd through the engine air supply

if the engine air cleaner a little clogged the engine can suck oil up past the compressor piston rings

and enter the wet (#1) air tank and gradully move thru the air system

we also notice some trailers have the main air supply line 1/2"

to charge trailer air system U need to hold the buttin in @ the cab control

where by a 3/8" supply line the button dose not need to held in as the supply of air is a little less demand from the truck

ifn U get my drift

LOL

cya

 

[mrsmackpaul]

I have never had or driven a truck with a air dryer

I did mention it on here once and all new trucks in Australia have air dryer fitted as standard, which to my way of thinking is a good thing

Having said that I now have asked about draining air systems as Swishman suggested and I was surprised at the amount of drivers that never ever drain their air system, didn't even know they could

Draining the air tanks is something I have always done, dunno who taught me to now but from before I ever drove a truck I new to drain air tanks, gramdfather or my father no doubt taught me

I found when in the more tropical parts of this wide brown land, during the wet season draining the air system is more than a daily occurrence as I see bubbling water at the breakaways (glad hand type connections) when it's really muggy and uncomfortable the air system on a truck feels muggy and uncomfortable as well and requires more attention 

The amount of crap that comes out of the air tanks, even on the trailer in these conditions is quite stunning, I guess it is in the air and condenses further back thru system when it stops travelling 

Some really good points there Geoff and I need to lift my game as I never really check most of that unless I notice a issue

Thank you

 

Paul

[Geoff Weeks]

The British and Austrian buses we got didn't have driers and use the discharge line unloader, so will not work with many (almost all) North American driers. With you guys getting so many Pommy and Europiean  stuff, that may play into why driers are not as common for you.

 The AD-4's I put on my stuff worked real well, Nothing out the wet tank, nothing out the trailer tanks. If I saw even a little moisture in the wet tank, I knew something was up. I had two trucks with AD-9's and they did ok, but when I saw moisture I knew there was a problem. Problem turned out to be the threads holding the element in the drier.  That is when the AD-9's got swapped for AD-4's. I could pick-up core AD-4's for about $20, kit em and be good to go.

 I never did yearly services, I just kept watch on the wet tank, and when it started to show moisture, it was time for some action. I had air start, so moved a fair bit of air though the driers over the years.  On the other hand, my trucks air systems were kept tight, so running down the road, there was little air put through the drier.