Air pressure found down

[Brad_SwiftFur]

Check your parking brake plumbing, there may be a leak when the parking brakes are released, which of course won't be evident when they are applied. A leaking "can" can go to a ruptured one without warning.

 

[cadillackid]

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.

the old AD-4 air dryers triggered governer cut-in and outs every 45 seconds or so as they used wet-tank air for the extended purge.. these were in use for MANY years and governers / compressors didnt fail at high rates.



the AD-9 air dryers reduced this issue as they use internal air for the initial purge and then leave the purge valve open for mopisture to evaporate throughout the whole governer off cycle.



I need to look into controlled response air valve for my DEV bus.. its an instant response now. . even just walking around in the bus triggers the valve to exhaust or pressurize with no delay.. so i get the every few minutes on / off of compressor while driving..



I know my tank capacity is somewhat small as my bus has the crappy 3-in-1 tank.. there is an added accessory tank but its not very big either..

 

[ABBus]

the old AD-4 air dryers triggered governer cut-in and outs every 45 seconds or so as they used wet-tank air for the extended purge.. these were in use for MANY years and governers / compressors didnt fail at high rates.



the AD-9 air dryers reduced this issue as they use internal air for the initial purge and then leave the purge valve open for mopisture to evaporate throughout the whole governer off cycle.



I need to look into controlled response air valve for my DEV bus.. its an instant response now. . even just walking around in the bus triggers the valve to exhaust or pressurize with no delay.. so i get the every few minutes on / off of compressor while driving..



I know my tank capacity is somewhat small as my bus has the crappy 3-in-1 tank.. there is an added accessory tank but its not very big either..

So your advice is to plug it back and forget it?

 

[Booyah45828]

the old AD-4 air dryers triggered governer cut-in and outs every 45 seconds or so as they used wet-tank air for the extended purge.. these were in use for MANY years and governers / compressors didnt fail at high rates.



the AD-9 air dryers reduced this issue as they use internal air for the initial purge and then leave the purge valve open for mopisture to evaporate throughout the whole governer off cycle.



I need to look into controlled response air valve for my DEV bus.. its an instant response now. . even just walking around in the bus triggers the valve to exhaust or pressurize with no delay.. so i get the every few minutes on / off of compressor while driving..



I know my tank capacity is somewhat small as my bus has the crappy 3-in-1 tank.. there is an added accessory tank but its not very big either..

I'm not aware of any system that functions as you've described. If an air system is cutting in and out every 45 seconds, something is wrong, and that frequent cycling will wear out parts in the system.

The extended purge ad4 dryers still had a check valve between the dryer and wet tank that prevented air flowing from the wet tank back into the dryer. The only difference between an extended purge dryer and a standard purge was the extended purge had a larger dryer housing, which would backflush the desiccant with more air then a standard dryer during the purge cycle.

 

[cadillackid]

I guess every old bus in the vintage groip with an ad4 has a bad check valve and the docs which state it uses system air for purge are wrong then but suit yourself

I have seen quite a few operate on short cycle esp those with the small 3 in 1 tank.

I first replaced my ad4 with a reman ad4, same operation.
I then replaced with an ad9 problem solved as also the same with a fellow vintage bus enthusiast who hated the short cycles..

The bigger busses with 3 separate tanks worked much better with ad4.

Luckily the ad4 is old and not likely found on what most have here. The ad9 is the defacto standard until the late model stuff with ad-sp which has its own purge tank( I think that’s the little one with attached purge tank)

 

[Booyah45828]

http://www.bendixvrc.com/itemDisplay.asp?documentID=2670

Tell me what page it says wet tank air is used for purge?

On page 3 on the right side of the document it states "The end cover single check valve assembly prevents air pressure in the brake system from returning to the air dryer during the purge cycle." I can't say it any better.

We've got a few in the fleet that still have the ad4 yet. They're old, and repair parts are near obsolete for them. One of them in use actually had the check valve go bad in it. I had to resort to installing a tank check valve in the delivery port threads to get it to function. To be honest, my personal bus still uses an ad4 dryer.

I don't know what people do with their old buses in the vintage bus group. But if they're purging every minute or so I'll gladly tell them they're wrong. If you are purging that frequently, something has failed to cause that to happen, or you've changed the air needs of the system and need more storage capacity.

 

[cadillackid]

So I had 2 bad ad4???

My AD9 works great and was an easy swap so I tell most anyone to swap their 4 for a 9.. eso since the dessicant is likely long expired in most busses when they are acquired ..

You can bash me all you want like you enjoy doing but my real world experience with the old stuff is what I go on.. oh and my bus purged every minute for many hours / miles without nothing breaking.. I upgraded the dryer when I couldn’t get the dessicant for the ad4 any longer and noted the change in behavior at that point..

 

[Booyah45828]

You must have had 2 bad dryers. Or you needed more storage capacity.

Like I said, the check valves are obsolete from bendix, so I wouldn't be surprised if the check valves in a reman ad4 were failed out of the box. The ad9 and ad4 are functionally identical. So your issue being resolved by the swap shows something in the old one being faulty.

If you'd like, you can try installing a tank check valve on the outlet of the drier and see if that remedies your frequent purging with an ad4. It did for the last one I worked on.

If the vintage bus group wants to act like an ad4 purging every minute is normal, more power to them, but that doesn't make it correct. My real world experience as a mechanic working on all things, including school buses, is what I go on.

No offense to anyone with what I posted. And as far as bashing is concerned, I don't enjoy bashing anyone, and bashing wasn't my intent. I was being critical of what you said because it was incorrect. Nothing more, nothing less. If that's bashing, I don't know what to do, or how else to correct false information.

 

[ABBus]

Quick update.
I plugged the sensor back this morning, smooth sailing today, and the gauge still went down but way slower. So I guess I worried for nothing (again), and the fast decrease in pressure was indeed due to the rear height valve acting crazy on the rough road. I'll probably replace this valve with a more reasonable one.