Air pressure found down

[ABBus]

Hello y'all

Issue of the day: the air pressure on the secondary tank goes down while on the road. Starts from 120, and then goes down to around 95 (takes 3-4 minutes) and then up to 120 in less than a minute. The other needle doesn't move (120 psi).
My first thought was to incriminate the front air bags, as it is the last thing I installed. So I stopped by the side of the road, stopped the engine and went out to see if I could hear some leaking noise. Nothing. Once back inside, I noticed the needle didn't drop! Started the engine again, needle stays still. Hit the road, and the pressure started to go down again.
Could it be the leveling valves, constantly trying to maintain the level on a rough road? Seems a lot of air to me, but I can't find another explication. Any idea? Thanks!

Edit: I hate when my smartphone tries to be smart and changes some words. The title was "Air pressure goes down"

[rossvtaylor]

Hmmm. The airbags should be pressurized by the auxiliary tank, which doesn't have a gauge, so a leak in an airbag should not show up on just one of the brake tanks (primary or secondary). Unless the aux tank is tied into one of the brake tanks, and not the main line, that shouldn't happen. We bought an auction bus with a rupture airbag and the only noticeable symptom was that the bus leaned and that it took longer than normal to build pressure in the brake tanks...but they built evenly.

EDIT to ask: Did you install airbags where none existed and, if you did, are they pressurized by the secondary tank? You really seem to know what you're doing...so I'm guessing not...but I had to ask.

[ABBus]

Originally Posted by rossvtaylor

EDIT to ask: Did you install airbags where none existed and, if you did, are they pressurized by the secondary tank? You really seem to know what you're doing...so I'm guessing not...but I had to ask.

I used the line which was going to the air seat, coming from the auxiliary tank.

[rossvtaylor]

Originally Posted by ABBus

I used the line which was going to the air seat, coming from the auxiliary tank.

Which is exactly right...and I guessed you did it right. Unfortunately, the issue remains a mystery...

[Booyah45828]

Chock the wheels, put stands under the frame, and then release the park brake. See if the relay valve in the rear is leaking. There is a double check function in some of those that will use secondary pressure to apply the rear brakes in case of primary failure. Some of those valves can get gummy/corroded and cause a pressure drop like that.

After that, spray and then jiggle all of the height valves for your suspension.

After that, verify the check valves are sealing between the drier and wet tank, and between the wet tank and secondary tank.

After that, your guess is as good as mine. But if you're loosing air on tank like that, it's for sure leaking somewhere, and needs to be fixed.

[joeblack5]

I am no expert on this but can it be that emergency brake line at cracked and start leaking whenever the rear axle moves up and down? I do not know if the emergency brakes are fed from the first it secondary tank.

Also if you have by chance an auto condensation discharge valve with a float. Maybe it is sticking when you go thru a hole on the road.

Good luck.

Johan

[cadillackid]

isnt it normal to see the pressure drop on the tank with the air suspension? the wet tank feeds the aux tank, when the wet tank goes down to 95 the governor cuts the compressor back in.. someplace around 95-100. if your system has a check valve between the primary and wet tank and doesnt have a feed from primary to secondary then you'd see primary stay the same because you arent using the brakes..



the air suspension drops down the wet and aux pressure and triggers the compressor.. once 120 is reached the air dryer purges and compressor cuts off


seems pretty normal?

[ABBus]

Originally Posted by cadillackid

isnt it normal to see the pressure drop on the tank with the air suspension? the wet tank feeds the aux tank, when the wet tank goes down to 95 the governor cuts the compressor back in.. someplace around 95-100. if your system has a check valve between the primary and wet tank and doesnt have a feed from primary to secondary then you'd see primary stay the same because you arent using the brakes..



the air suspension drops down the wet and aux pressure and triggers the compressor.. once 120 is reached the air dryer purges and compressor cuts off


seems pretty normal?

Yes, maybe (hopefully).
But I never noticed such a "quick" drop in the secondary tank before.

[cadillackid]

fro mwhat ive seen school busses all have no-delay levelling valves for the suspension. meaning as you bounce along the levelling valve constantly exhausts and then draws from supply for air.. the worse the shocks the more the bounce.



my bus can sit for a day and only go from 120 to 95 or 100 but driving it ill watch my air drop from 120 to 95 in a matter of minutes and then my governer cuts back in..

[s2mikon]

I think that your front air springs are the source of air useage. If you can find a smooth road you will know.

[cadillackid]

Originally Posted by Booyah45828

Chock the wheels, put stands under the frame, and then release the park brake. See if the relay valve in the rear is leaking. There is a double check function in some of those that will use secondary pressure to apply the rear brakes in case of primary failure. Some of those valves can get gummy/corroded and cause a pressure drop like that.

After that, spray and then jiggle all of the height valves for your suspension.

After that, verify the check valves are sealing between the drier and wet tank, and between the wet tank and secondary tank.

After that, your guess is as good as mine. But if you're loosing air on tank like that, it's for sure leaking somewhere, and needs to be fixed.

I had that very valve leaking on my old superior.. you could hear the air if you released the park brake with the engine off. wasnt very expensive and was easy to change..

[ABBus]

Originally Posted by s2mikon

I think that your front air springs are the source of air useage. If you can find a smooth road you will know.

I'll try to disconnect the leveling valves and secure the arm.

[s2mikon]

That will simulate a smooth road. At least for the air springs. Did you use the quick reacting level valves or the standard ones?

[ABBus]

Originally Posted by s2mikon

That will simulate a smooth road. At least for the air springs. Did you use the quick reacting level valves or the standard ones?

Haldex 90054007. Standard one, I guess ...

[s2mikon]

Originally Posted by ABBus

Haldex 90054007. Standard one, I guess ...

That is the Controlled response valve. The one we use here in the oilfields.

[s2mikon]

It may be that the total volume of the bags may be on the small side and needing constant adjustment. Ping tanks anyone?

[s2mikon]

I switched to those for the rear of my bus due to some of the fine NM roads. Lots of small bumps from the first layer of pavement lifting off would drop my air pressure. As soon as I hit smooth road I had stable air pressure.

[Booyah45828]

With a controlled response valve installed, you shouldn't see a bunch of air loss from normal road travel. The cr type valve is designed so that the constant add/discharge of air from a bumpy road shouldn't happen, bad shocks or not.

I guess if the road is super rough then you might see that. But loosing 30 psi in 3 minutes is a bunch, and if that's normal operation, then I'd be looking into installing more tank capacity. A governor cutting in and out in 5 minute intervals is too frequent IMO and will lead to part failure sooner rather then later.

[ABBus]

I disconnected the rear height sensor (the rear valve doesn't look to be a controlled response), and that solved the issue. I guess I should replace the valve with a Haldex.
For now I'll let the valve disconnected, don't want to burn the air compressor.

[s2mikon]

Be careful of driveline angle. It is a damn short shaft.