[John]

Does the tag dump it’s air? On early years they had what was called tag logic, when aux air got low it dumped the tag. Don’t remember the low pressure number for this to happen, maybe in the 45 psi range. What year is your chassis?

[Randy]

when aux air gets low the tag will dump like tom said

[Dick]

Mine (1984) does this. Anyone know the logic behind having this “logic?”

[Archived]

My 96 (95 chassis) has the “old” tag logic and does the same thing. It will dump once AUX air press goes below about 20psi. Which from what I’ve heard from others is lower than some. No idea why.

There was a service or maintenance bulletin posted here awhile back about this very thing. Three “kits” were once avail to convert to newer tag logic which doesn’t dump when AUX air bleeds off. Kits are/were based on chassis config and VIN range. I called Prevo Parts regarding these, I think all three were in the $400-500 range and were not in inventory. Parts guy was going to check some other places and get back with me. He didn’t and I never followed up either.

The info that was posted here listed each kit and respective contents.

[Bill]

My 95 chassis does this and I wish it did not.

I told Jeff a couple of years ago that IF I knew the valve involved, I could put a hose in the exhaust port and run it back to the bumper. If it needs to exhaust while going down the road, it could.

When I stopped for the night, I could shut a valve at the end of the hose and lock the air in and keep the tag in the road position.

He gave me a look that I interpreted as, “please do not breathe your stupidity on me.”

That was even before COVID!

[DON]

fixing aux air leaks will help

[steve]

Thanks for the responses and I bet that is exactly what is happening. I have noticed the Aux system losing some air so it all now makes sense.

I will take on the Aux leaks and maybe see what I can learn about the kits as well.

[Archived]

When the Aux tank pressure drops and the tag “interlock” Norgren valve losses it’s pilot air pressure, the Norgren spring moves the shuttle to isolate the drive air bags and vent the tag bags. The problem with this is there are two Norgren valves and so the right and left side will vent slightly differently resulting in the coach going out of level – if parked and not on the air bag stops.

I plugged the vents on these 2 Norgrens. Now that means I can not raise the tag. But I never raised the tag anyways.

[Archived]

Do what Joe Carter said, or do Prevost SB96-14B

Up through 1996-1997 the plumbing of the tag axle lift circuit required that there be sufficient air pressure in the auxilary air tank air sytem to properly pilot the valves associated with the tag axle and tag lift system. Thus you needed good aux. air pressure to keep the coach at normal ride height in the rear of the vehicle. Lack of proper air through an air leak would eventually reduce pilot or control port air pressure to the point where the internal spring tension of the valve overcame the control pressure and the system cycled much the same as if you had flipped the tag lift lever. The difference being that there is no aux air pressure left to do anything with the system.

Engineering at Prevost redid the pneumatic “plumbing” of this circuit in 1997 so that there were no control pressures to any of these valves to keep the tag axle down and the tag lift circuits off. There was a maintenance bulletin released “Service Bulletin 96-14B) to increase the reliability of the pneumatic system. The bulletin can be found at

https://secure5.volvo.com/technicalpublications/pdf/Sb96-14B.pdf

There were various kits made at the time for different models. These would come with instructions and valves that had the proper brass fitting for the application. The kits also included an inversion valve that had to be plumbed into the circuit. Prevost has always cancelled the brake boosters on the tag axle when it was lifted to prevent tire damage. As we were changing the way the axle was controlled we had to change the control of the Bendix R12 valves that controlled whether there would be brake application pressure sent to the boosters on the tag axle or not.

The modification takes some time as the old valves need to be removed and air lines properly identified. Then the new valve installed correctly and the inversion valve installed. Plus the light circuit in the dash had to be changed to reflect the lack of air pressure in the controls when the vehicle was in the normal ride height position.

All that being said: to check your system! Run vehicle and get all air tanks filled. Turn off coach. Open accessory (auxilary) air tank and bleed down the pressure. If the coach has the newer system installed the vehicle should NOT settle in the rear as if you flipped the tag lift lever. If if does squat, then it most likely did not have this modification done.

[Archived]

Thanks Don.

This was new to me as I had missed the old conversations about this issue. It did not happen during the first 5 years of owning the bus so clearly I have “sprung” an aux leak that is likely the culprit.

I am going to check into leaks over the next couple days which i should do either way, then I’ll start to look into SB-9614B and see about kit availability and if anyone is doing them (I can’t or don’t get under the bus so nothing I can do) these days.

We do raise the tags to improve our turning radius so ideally I can just eliminate it either via leak prevention or the SB.

Nobody raised it, but I assume that in the meantime having it sit on the Drives after the tags release (it does not lean BTW) is not going to cause other issues.

Always learning….

[Chad]

I just replaced my original Inversion valve, here ia a picture, and a schematic how it is plumbed

---

1984 96" with 8V92TA

(remodeled Liberty Coach)

Greenwood, MO

[Bill]

Just to add some verifying experience – and to thank everyone for posting so generously on topics of interest and utility for me as a new owner.

The rear end of our 1995 Marathon started ‘squatting’ after 2-3 hours. Thorough online reading of this forum led me to check the Auxiliary air system as the cause, as the tag was deflating using the old ‘illogical/logic’ system for the rig’s age. So off we go to find leaks in the auxiliary system starting with listening then progressing to soapy water to pinpoint. Thankfully few places on this rig are potentially involved – Pneumatic belt adjusters both ok; ISRI seat 1 leak at a compression fitting – re-terminated and leak gone; no apparent (?) leak in step cover; Pressure regulator valve above norgren stack for level-low = home run. Leaking like a sieve. Replaced valve. No slides, toilet is electric only, no pneumatic pocket doors. No more squatting after 2 days — oh yeah. Since the air was being exhausted from the tag (and I heard it do this and was able to replicate easily), I knew what the issue was/is.

1 down – now – leaning to the left overnight but no squatting or tag exhaust. So – re-reading again for fortification, then off we go again. L rear 5-port Norgren leaking substantively, L rear leveling valve leaking within valve (not a fitting). Parts on order – will replace and advise.

Thanks for making this easier than it could have been – at least for a start. I’m under no illusion that the leaks are “fixed” and there won’t be others. in the future or even more to find right now..but I did want to say thanks to everyone for pointing in the right direction and helping with the confidence to begin tackling the problem.

John

[al]

When I 1st got bus it leaned after a few hours and finally found tag bag had a rotted spot and air leaked where it rolled around so I replaced all 8 bags leans no more. How old are your bags?

[Robert]

The best answer is “I’m not really sure” except for 1 which I had to replace. Right forward bag on drive axle is new.

The rest “appear” to be 2-3 years old per shop where I took it. That said, there is no documented replacement date on any of them besides the one.

I know that in the end I may have to replace the remaining 7 bags sooner rather than later, but I am relying on the shop where I took the rig (known to me and reliable) not to steer me too far astray. They are not known for doing anything on the cheap, and I do trust their assessment as a general rule. I told them that I was prepared to replace all the bags and valves just to start with a clean slate, but they didn’t think that was called for yet. The bags could easily be the source of the problem, however, especially when you inflate the bags fully and the areas prone to cracking at top and bottom are visible. I promise not to let this go too far afield as Jon has noted repeatedly that one can replace the bags and Norgrens several times over for the cost of ‘chasing’ leaks versus starting with known good configuration.

Thanks for the timely reminder.

John

[Art]

Which air bags did you install? The ones Prevost lists as correct or the ones that are listed as correct on this site?