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Separation between Chuch and State
OK, really separation between the 2 main air tanks and Aux air.
In my last bus I could drain the aux tank and the main air tanks remained at a full 120#. This coach has the Thomas air compressor with its 2 gallon(?) tank and then, I assume, the actual aux tank under the bus between the front tires somewhere.
With this coach, when parked, whatever pressure I set on the Thomas aux air pump to it what pressure the entire system sees. So, if I drain the air from the Thomas air tanks the main/aux air pressure drops to zero as well. In the past I only kept the aux tank pressured up to around 50# to keep the tag in place and the air door lock working. If I do that with this coach the 2 main air tanks drop to 50# as well.
It seems like I am missing a check valve somewhere or the valve has failed. If so, any idea where I would start looking (no pit and no jacks to go under the coach). If that’s normal then do you all keep the Thomas setup for 120# or just let the mains draw down. It doesn’t seem like it should be normal or safe as a loss of an aux line would let all the air out of the mains as well.
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26 Replies
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Bill I’m gonna try and answer your question(s). I will do it in three separate replies. I got this info, saved it to my log book, here on PC from “Prevost Service”. I hope this helps.
#1 – Air System Info
Let’s start with where the air starts; the supply system. The heart of the supply system is the engine driven air compressor. It feeds an air dryer that removes moisture and oil. The dryer feeds the wet tank and a return line to the compressor mounted governor. The governor signals the air compressor to stop producing pressurized air and the dryer “sneezes” to unload the pressure line when the governor set air pressure has been met.
The wet tank then supplies air to the front and rear brake main tanks and the auxiliary tank. To isolate the front and rear braking systems, air entering these tanks does so through a check valve at each tank. Once air is in the main brake tanks it cannot escape back into the supply feed.
The auxiliary air system cannot have the same priority to supply air as the braking systems. To ensure the braking systems are fulfilled before the auxiliary system, the supply line is fed through a protection valve to the auxiliary tank. The protection valve opens when either side of the valve reaches a specific pressure which is somewhere between 60 and 75psi. If your coach’s air systems leak over time you may have noticed that air builds in the front(secondary brake) and rear(primary brake) systems first. At the rating of the protection valve you then hear the rush of air entering the auxiliary system.
That is basically the entire supply air system. It supplies all 3 main air subsystems, rear braking, front braking, and auxiliary. Once those systems are filled the governor effectively turns off the compressor and each of the 3 systems are protected from their air returning to the supply air system.
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#2 – More Air System Info
Burketts valves were what Prevost used in the mid to late 90s as a replacement for the Norgren three way valves. They come either NO or NC and they cannot be altered. The first versions of the valves did not have a return spring in them and we had numerous issues where the valve would stay in the midway position. This led to air systems not building air correctly. Normally the bad valve was on the tag lift system and by simply cycling the system you could get the valve to seat itself and the air would build correctly. Burkett came out with a valve with a return spring ( the new Burkett valve) but we went back to Norgren valves eventually. Another issue with the Burkett valves was they were made for a much cleaner, industrial application. The coach pneumatic system in not a completely clean and dry environment. The Burketts did not fair well on the coach. The Norgrens were more robust and did not mind a little oil and water in their working environment.
Most of the people in Prevost service do not know anything about the Norgren numbers. Prevost parts secures the correct Norgren number for the application from the engineering department and then a Prevost part number is generated. To get the Norgren number we would have to get someone in parts to look at the cross reference file for the correct Norgren number. That is why you may get a blank stare from a tech if you come out with a Norgren part number.
As noted also above the type of valve you need (NO or NC) will be dependent upon your pneumatic configuration in the rear of the coach. 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.
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#3 – Tank Pressure Protection Test
The tank draining sequence is: to bring the entire coach pneumatic system up to pressure. Next, drain each tank starting with the wet tank. Drain the primary next. The secondary next. And finally the accessory tank. You are looking for moisture, but perhaps more importantly you verify all the protections in the system are functioning. That is very important because if each tank in succession has air pressure when the preceding tan has none it means a catastrophic loss of pressure in any system will not leave the remaining systems without pressure.
The DOT brake check will verify the protection valve between the secondary and accessory tanks is not allowing brake system pressure into the accessory system until brake pressure has been satisfied with at least 75 PSI.
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To avoid confusion, to the best of my knowledge there are no Norgren or Burkett valves within the Prevost pneumatic system.
All the valves of that type are after the Prevost accessory tank and except for converter purposes, they are mainly used for control of the suspension, to tighten belts, etc.
The check Don describes that calls for draining the tanks in sequence is something that should be done every time someone is beneath the coach because it is verification the pneumatic system and its protections are in place and working.
It is possible a problem with the parking brake and its pneumatic components can drain both the primary and secondary tanks simultaneously. But it is important to remember a DOT check will provide any information that is not available by just draining the tanks.
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Prevost bus air systems have many Bendix brand parts. Example is the PR-2 pressure protection valve on page 26 of this Bendix catalog: Catalog
My bus uses the 75 psi PR-2, # 277227N. -
DOT check? No access to the tanks as I have no (safe) access under the bus. I did finally fly back to TN and get my bus out of hock from ETLC and drove back to ND. The temps have stabilized and we are hoping we don’t see another blast of 20 below temps!
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No getting under the bus.
Air Brake System Check
For air brake equipped vehicles, the proper procedures for inspecting the air brake system are as follows:Test Air Leakage Rate (Static check)
With a fully-charged air system, typically at 120 psi, turn the engine off, chock the wheels, release (push in) the parking brake button and trailer air supply button (for combination vehicles) and time the air pressure drop. After the initial drop, the rate of pressure loss should not be more than 2 psi per minute for single vehicles and no more than 3 psi per minute for combination vehicles.Test Air Brake System for Leaks
Release (push in) the parking brake and trailer air supply button (for combination vehicles), and apply pressure to the foot brake. After the initial drop in pressure, the rate of pressure loss should be no more than 3 psi per minute for single vehicles and no more than 4 psi per minute for combination vehicles.Test Low Pressure Warning Alarm and/or Signal
Turn the key to the on position. Rapidly depress and release the foot brake to reduce air tank pressure. The low air pressure warning signal must come on before the pressure drops below 60 psi.Check that the Spring Brakes Activate Automatically
Continue to rapidly apply and release the service brake pedal further reducing air pressure. The trailer air supply button (for combination vehicles) and parking brake button should pop out when the pressure falls to usually between 20 to 40 psi. This should activate the spring brakes.Check Rate of Air Pressure Buildup
While the engine is operating at 1800 RPM, the pressure should rise from 85 to 100 psi within 45 seconds in dual air systems.Test Foot (Service) Brakes
With normal air pressure, release the parking brake and trailer air supply button (for combination vehicles), move the vehicle forward slowly at about 5 mph and apply the brakes firmly using the brake pedal. Test for any pulling to any one side or delayed stopping action. -
Thanks for the DOT info. I will have to give that a try when parked at a site that does not have a cliff behind me!
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Bill, Here is free advice, worth every penny you pay for it.
If you do the test as Don provided you have done 1/2 the checking you get to do beneath the coach. The second reason for draining the tanks is you are doing a moisture check. Assuming the system is not flooded it is probable the only two places you would expect to see moisture is the wet tank and the accessory tank.
If you maintain the air dryer you likely will never see a drop of moisture in the wet tank or the primary and secondary tanks. The accessory tank can get moisture if you do not have a good quality water separator or dryer installed downstream from the aux compressor. If you do run the aux compressor keep in mind it will produce water and your vigilance is the only thing to prevent that moisture from migrating into the accessory tank, and then into accessory systems.
When I am using my coach I can collect moisture produced by my aux compressor in a water trap, and I drain that as required to keep moisture from my accessory tank and systems. If I failed to keep water from my accessory system I could introduce water into my air bags and throughout myNorgrens which shortens their life.
I change my air dryer every two years and if I ever get anything from my wet tank it is dust. The air dryer will protect the entire bus pneumatic system very well, but it does need maintenance.
So if you cannot get beneath the coach (I do it every 5000 miles) keep up with the maintenance of your dryer and moisture traps and you will be OK.
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I had one Burkett 3 port valve under rear of bus on driver side inside frame above tag used for lifting tag axle , and I replaced it with a 3 port Norgren valve. Those valves do the same thing they can be used as NO or NC,much better valve. As per check valves,I replaced both of my check valves a couple of years ago and now the mains stay put. 1st picture was burkett valve now Norgren 2nd is check valves.
2000 Featherlite H3-45
2024 Jeep Grand Cherokee
Leesburg, FL -
Always interesting information but does anyone know if the main tanks should drain when the Aux tank drains? If not, where should I be looking to figure out why this is happening?
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Brake and wet tank should not drain with aux, check valve.
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Bill, there are check valves as well as a protection valve that will come into play if the accessory tank loses air.
On the Prevost car web site accessible through the parts and service link are pneumatic diagrams for your coach that show both the location and type of valves and components that will prevent all primary and secondary air system pressure losses.
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I spent a lot of time starring at the air schematics and studying the flow of air from compressor to dryer to wet tank to Primary and secondary tanks and to aux tank it shows check valves protection valves it is pretty nice to see this, I understand the way the system works and you will see when wet tank fills also 2 primary and tanks in rear of bus and see check valve and same time it fills secondary tank and aux tank but it has a delay from filling from protection valve at aux tank so main tanks will reach 75 psi before aux tank fills. I was going to install a wet tank psi gauge but I realized wet tank and aux are the same psi till 75 psi then wet and aux seperate from protection valve so I did not install gauge. Driving down the road if your aux gauge drops from 120 psi to 95 or 100 psi and main tank gauges stay at 120 psi your leak is most likely in aux system. I had a pretty good leak in 1 of my air rams (foot well slide) and driving I watched my aux gauge cycle every 5 minutes or so and every time that happens the compressor and dryer is cycling and purging so now driving down the HWY no brakes being used all 3 gauges stay at 120psi. Now if you have a leak in braking system all 3 gauges drop. Also look at level low system and drive mode all air is coming out of aux system.
1984 96" with 8V92TA
(remodeled Liberty Coach)
Greenwood, MO -
I have only 2 air gauges on the dash for the 2 brake air chambers. The gauge for the Aux tank is viewed in the outside drivers lower door.
The 2 main air tank gauges never move when driving and not touching the brakes. When parked, they lose air in unison.
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