[Archived]

Gil,

If my understanding of what you are saying is correct you might have bigger problems than a simple rebuild of the dryer.

You mention the aux air compressor runs a lot suggesting in your description that the air from the aux compressor is leaking out the dryer. If that is what you said, and in fact aux air is leaking from the dryer then it means you have issues with the check and protection valves intended to prevent exactly that from happening.

Briefly, if you air up your coach, you should have the ability to drain air from the aux air tank (draining the entire aux air system) and still have about 75 PSI in the secondary brake tank. Drain that and you should have pressure in the primary brake tank.

In other words each system in the coach can experience a catastrophic failure and protection valves or check valves should prevent air loss in other systems. I hope I am saying that in a way that makes sense.

If the aux compressor runs an inordinate amount of time, and is capable of pressurizing either of the brake systems (hence the long running time) it is more important you treat the system protection valves as a priority, but also deal with the dryer. Dryers can be bought rebuilt, or you can buy kits to rebuild them.

If you do get someone to work on the air systemsmake certain they know what they are doing.

[Archived]

The aux air compressor running a long time should have nothing to do with the brake tank systems as Jon has pointed out, including the dryer. If the purge valve is leaking at the dryer, the result would be a depletion of air in the wet tank. Depleting the wet tank air should have no effect on the primary and secondary brake tanks.

I’ve had these problems before:

Aux air and brake air depleting at the same time. Had to replace the pressure protection and check valve in the steering compartment. This was on a 94 Liberty and was probably the same setup as you have.

I had an air dryer on my 97 Royale that leaked similiar to what you described. The fix for that was to rebuild the purge valve.

[Peter]

Tom,

Your pictures are always a great help. The second picture shows the purge valve removed and next to it a black rubber flapper over what I believe is the turbo valve. It’s that vale that is leaking.

This morning I went out to the coach and did the following:

• Noted the pressure of the main air tanks to be 58psi.
• I ran the aux compressor up to it’s cutoff of 106 psi. The main tank pressures did not change and the air dryer did not leak.
• I started the engine and it took a bit before any air pressure changes occured. Then the main tanks began to raise in pressure until they cut to the cutoff of 115psi. The aux also rose to 115psi (I think). Still no leak.
• I turned off the engine and drained the aux tank down to nothing. The main tanks remained at 113 psi. So, the protection valves appear to be working. Still no leak.
• I turned to aux compressor back on with the engine still off. When the aux pressure hit about 80 psi the leak came back and the main tank pressure remained at 113 psi. I turned off the aux compressor and when its pressure dropped to about 60 psi the leak stopped.

I’m thinking my protection valve between the mains and aux is working. I don’t understand the theory of operation of the turbo valve (if that is what it is) to know why it’s not functioning properly.

Another tidbit. The main tank drain in the passenger side engine bay does not work. Air does not escape when I open the valve except for a very minor flow easily stopped with your finger. The same when pushing the valve stem fill valve.

[Andre]

Gil, by main tank drain valve I presume you mean the fill / drain valve on the rear passenger side post. It is possible that has some built up crud that is inhibiting flow. It might need to be removed so the line can blow itself clear. It is not uncommon to purge some oil along with moisture when opening that valve.

It sounds like the protection valves / check valves are working, but if so, there should be no way aux air pressure, regardless of what pressure it is can impact the dryer.Have you tried fanning the brakes to get the primary and secondary pressures to zero? Then running he aux compressor to see if you hear escaping air? To verify aux air is not backflowing?

I don’t know how your coach is equipped, but behind the tag axle is a Prevost installed pressure regulator that has a few items that come off the inlet and downstream side of it and its related valving. The regulator is supplied to provide air to the transmission retarder. From that point air also runs to the belt tensioner pressure regulator. Can you be hearing that instead of the air dryer which is forward of the drive axle? Could you hear an air leak associated with the tag axle three port valves located up on the frame between the drive and tag?

It sounds like you need to be beneath the coach while someone runs the system through the various pressures.

[Archived]

Jon,

Yes, the clogged line is the one going to the engine bay. I’ll have it removed and cleaned out.

While the leak is occurring I went under the coach and most definately felt the air coming from the black rubber flapper pictured in Tom’s second picture.

I’m going to my local CAT service center in Ocala, because it’s close and I’ve used them before, next Tuesday and have them check the dryer. I’ll post their findings.

Thanks,

[Archived]

It still bothers me that your aux compressor can make the dryer leak air. Either I am misunderstanding what you are saying or something is wrong. Everything else I previously mentioned is on the aux air system. I can understand air leaking from it. I also marvel at the length of your arms because I couldn’t get my fingers on any of my air dryers without being under the coach or having my body under it.

[Archived]

Not sure what the parts are called Gil, but on mine, when the
purge would happen, it just wouldn’t shut off and continued to leak
air for some time. Even with bad ears I could hear it leaking and it
would eventually quit.

While you’re at the shop, have them turn up your compressor
governor to 120 – 125. Sounds like yours is cutting off at 115.
Another 10 lbs helps when leveling or raising the bus.

Also, I get rid of that shrader valve in the rear when I get a bus
and replace it with a quick coupler.

[Archived]

Great idea on the quick connect valve Tom!

Jon I was under the coach. No, I don’t drag my knuckles when I walk 😉

[Gerald]

Tom, wondering about your method of using a quick disconnect instead of the schrader style fill.

Did you make up a double ended male adaptor to join the coach to the airline.

Seems one would need to carry that along so that if you required on the road service, air could be supplied without searching for parts and pieces. I know every time you go to Prevost, standard opp. is to connect to the air system. If they have to make up an adaptor, the clock is ticking.

Just sayin, JIM

[Archived]

The few times I have been to Prevost they never hooked me to electric or air. I have not had any lengthy repairs done so that may be why.

The fitting can be converted to a normal male fitting if a ball valve is placed in the line so it only gets opened when filling from that port. I think it is a great idea and putting an air fitting instead of a Schrader valve at both locations (front and rear makes a lot of sense.

Now you all know why Tom is running PC. He is the brains. I am the beauty.

[Archived]

I have two male ends with a mini gate valve in the middle and never leave home without it. Try filling 4 air tanks using the shrader valve and you’ll get the picture.

Jim – On a trip to Prevost Nashville, I didn’t even need to bring out my adapter, they already had one made up and prefer that method as opposed to clamping on an air chuck hoping it will stay.

Jon’s being kind, he probably starting doing this in 1987.

[Archived]

I just wish I could take credit for that one.

[Archived]

Very good Tom, I also like the idea and brought up the adaptor topic so that anyone else making the mod. would be aware of its necessity.

JIM

[Davy]

Problem fixed! The problem was the Turbo-Cutoff valve. The upper (main) oring was visibly flattened. Replacing this valve is easy with the dryer still installed. The repair kit even comes with a small cup that aids in pushing the piston up past the sharp opening.

While having that work done I have the purge valve replaced and dessicant filter replaced. These require the dryer to be removed, which looked to be a PITA. The mechanic and I had the same thought at about the same time–why is the dryer installed here? My last plastic coach had the dryer installed in the engine bay. If I had more time I would have had the dryer moved to the engine bay where the jack currently resides. There’s plenty of room if you don’t have Prevost Bus Air.

I also solved the mystery of the drain located in the engine bay. This is a dryer drain, so it will only drain when the dryer is below it’s purge pressure.

Has anyone extended the wet tank drain to a place more readily accessible? It would seem to me to be an easy retrofit to extend a hose off the wet tank to the engine bay or tool box bay and locate the tank drain valve there.

[Roger]

Gil, I did not engineer the Prevost coaches so I can only speculate on things similar to those you mention.

I have serviced all my dryers which I accessed from below. The first one was the worst with a snap ring of about 1/4″ round rod, doing that while laying on a barn floor. The spin on cartridges are much easier, especially since I have the benefit of a pit. Their location compared to in ther engine compartment is not as convenient, but there is a good engineering reason for the location. The dryer and the length of hose are routed and located probably because it is important to get some of the heat from the compressed air so the dryer will separate the moisture. The dryer is relatively ineffective if the air is very hot because the moisture really isn’t moisture at that point of discharge from the compressor.

That’s my story and I am sticking to it.

Some owners have installed lanyard operated drain valves on their air tanks, but the reality is that if you have the coach lubed at the suggested intervals, and drain the tanks while you are under the coach you probably are never going to experience any moisture problems. I doubt if daily tank draining would even make the air any dryer in the systems.

I might be wrong but a number of CC owners have complained about air dryer placement, but at the same time they usually attribute the difficulty to the placement of the little CC AC condenser in the way. (OK, I admit, I was puling your chain about the little condenser, you made it too easy with the comment about OTR)