When I start the engine, it takes only a couple minutes for the 2 air pressure dials to reach 120 PSI. Driving on a level highway at 60 MPH, not using the brakes, the pressure slowy drops to 105 PSI in about 8-10 minutes. The compressor then kicks in and raise the pressure back to 120 PSI in less than a minute. Both dials act the same way. Is this normal? Shouldn't the pressure stay constant if I don't use the brakes?


Also I read in manual on air brakes about air dryer and purging the system. The manual was taking about letting the air go by pulling a chain. I've never done anything like this. What do I have to do when dealing with my Barth air braking system?

Thank you

I can't say if it is normal for your specific equipment or not, but I would be suspicious, and perhaps alarmed.

My Cummins holds a steady 110# all time unless I am using the brakes.

Commercial equipment (trucks) had/have valves with chains on them to pull, letting the escaping air blow out any water accumulation in the tank. Prevents tank rust and system moisture vapor contamination. I have never seen the 'chains' on a motorhome, but I had cables added to my valves. But it be two years since I remembered pulling them.

The air dryer should alleviate the eed for pulling chains in light duty service such as a moho. The dryer is a dessicant that traps moisture from going into te air compressor, i.e. air dryer. The loud "pishhhh" sound that your bus (and trucks) let out for a second every few mintes when pressure is at normal is the air dryer purging itself of any collected moisture.

Since this only happens when air pressure is at max, the air dryer is higly suspicious regarding your pressure problem. It sounds like your pressure builds to 120# and the first time the dryer tries to purge, it dumps far too much air, down to 85#. So that is where I wold start looking for your problem.

But remember IT IS NOT FUN TO LOOSE AIR PRESSURE WHEN DRIVING so have it checked out!

You air system on the Barth is probably good. Taking about 2-3 minutes to fully charge after overnights rest is good and show that the system pump and dryer is in good shape.

This is a normal decline of air pressure during driving because it is used to power some vacuum generators (one in front and one in the rear). These are Bernoulli devices that require air passage to generate vacuum.

You Barth uses vacuum to power the actuators in the front heating system and also power the cruise control, you may have a small leak in the vacuum system that requires the vacuum generators to run more often. I have noticed on mine that when I don't use the cruise control, the decline in air pressure is a lot slower.

There are small shut-off regulators that are supposed to shut air off to the vacuum generators at about 50 PSI or so. That is why you have probably noticed that in the morning the air pressure is about 50-60 PSI. Mine will drop fairly quickly after shut-down to 60 PSI and then stay there for days with a very gradual decline.

You also may notice after shut down, hearing an intermitant small air noise from both the front and rear of the Barth, this is the vacuum generator devices working.

I am working on an electric vacuum system to replace these as I don't like the idea of exercising the air system constantly while driving. Besides the normal wear and tear on the air pump, it will shorten the life of the air dryer element and can't help getting more moisture into the tanks.

Thanks for the explanation Ed.
Your write-up refreshed my memory on the fact that I had taken out the venturi that controls the heather flaps in the dash because I tought it was defective. I am probably loosing air pressure there more than normally.
I will put it back and check.

Ed,

Did not mean to step on the toes of a Barth expert, but having my air pressure fluctuate to 85# while underway would make me very nervous indeed.

I have always wondered about my hvac controls. I have no idea how my heater-a/c controls get their operating power. I neither hear nor see any pumps. But they 'appear' to be the GM style controls, one would presume operates from vacuum.

Hi Garry, no worry at all, all Barths are indeed different, albeit some things are the same. If it were not a Breakaway, I don't think my assessment would correct. However, If Ray has removed the venturi generator in the front, he would still have air loss to the venturi generator in the back.

So Ray do you have a Dana vacuum operated cruise? As I mentioned in my previous post, I notice air loss when I use cruise control but not a lot when just using the heater because once the flappers are set, the demand for vacuum is low. I also think I have a vacuum leak in and around my cruise control but because I am going to change over to an electric vacuum pump I haven't been very worried about it.

The air gov on your coach is typical. Air psi cutoff is between 115 & 125 lbs. It has a 15-20 lbs kick in psi. So if it kicks off at 120 it will kick back in around 100-105 lbs.

The air compressor only knows to build air and then to kick off. A lack of air psi to the gov causes the air compressor to build. Once it reaches the set psi it sends an air signal to the compressor causing the unloader ports to stop building the air psi inside of the compressor.

This air psi stays in the line from the gov to the head of the compressor. Once it drops down to the "cut in psi" air is expelled from that line and the air compressor starts to build up all over again.

The fact that your air psi fluctuates is not that big of a deal. If it stays between 125 and 95 psi then you're OK. If your air psi can build up at an idle, running down the road at a faster engine speed will be easy for it. A little leak will be tough to find - you could also be using air for vacuum operated items. A diesel need a vacuum pump and a lot of coaches use a air psi driven venturi vacuum pump. This will use your air psi and could be the cause of your air slight air loss.

Thanks Bill ,I fell better. Once I put back the venturi I'm pretty sure I won't notice such a rapid change in pressure.
What about normal care of the air braking system. Anything I should be doing? I read about pulling chain to purge. What about the dryer. Anything that needs to be replaced on a timely basis (filter or such)?

quote:

Originally posted by Raymag:
Anything that needs to be replaced on a timely basis?

The air compressor has little maintenance needs. If you're looking for money to spend and you want to change something out replace the air gov.

If your air dryer is over 10 yrs old, change it out. The air dryer is nothing more then a desiccant cartridge. It takes the hot compressed air and extracts moisture out of the line.

Just when the air compressor stops building, the same signal that tells the unloader ports in the compressor to "Unload" that air psi signal is sent down another line to a dump valve. Once the dump valve opens inside of the dryer it purges the desiccant cartridge and dumps any moisture that is inside of it.

Most air compressors get the fresh air supply from a hose that runs over to the air intake. Some get their fresh air from a filter on the side of the compressor. You'll need to replace the air filter every other year if it's the latter.

Drain valves come in many shapes and should be drained on a regular basis. You could have a ¼ turn valve, you could have a pull cord, you could even have an automatic bleeder. You'll need to look at the lowest point in the tanks and see what you have. Look and find each tank and drain valve.

Some tanks have 2 drains. This does not mean that you only need to drain one - this normally means that the tank is two tanks. Look very closely and chances are good that you'll see a bead welded all of the way around the middle.

Another sign of two in one tanks is a jumper hose running from the front to the back with a check valve on one of the two sides. Your air system is split in two. The primary and secondary system air system, this is why you have two gauges.

The first tank inline from the dryer to the air tanks is called your "Wet Tank" that tank is the only tank that needs to be drained on a bi-monthly basis. 2 seconds open and you're done. If the wet tank does not have a pull cord on it, change it to a pull cord style drain valve to make your life easier.

If you see nothing coming out of the "Wet Tank" then do not waste your time draining the others until you change the oil. If, on a regular basis, you see nothing out of the wet tank, then go from twice a month to once a month drain interval.

Now, after all of that, there are other things that need to be looked at. Does your dryer have electricity running to it? Yes, Then it has a heater element inside of it. If you don't run in the winter then unplug the fuse for it. Plug it back in for the fall/winter months.

That unloader line I talked about earlier. Make sure it goes downhill to the dryer. If it drops too low and then goes back uphill to the dryer you will have moister pooling in the lowest point. This will refreeze or turn to sludge. From gov to dryer it should go downhill - never uphill. The only place it is acceptable to go up is within 6" of the air gov. and that's only "IF YOU HAVE TOO".

Looking to spend more on your air system? Place a 150-180 psi blow off valve in the head of the compressor. If your air dryer freezes, this will save you the cost of blowing out the headgasket on the compressor, or the air compressor discharge line.

Not all coaches or trucks have an air dryer. All coaches do have an air gov. and drain valves. If you have one without a dryer I would recommend that you get one installed.

Sorry to be so long with the answer. I would rather give you more of a response then less. Any other questions will be answered so please ask. The more questions, the better it is for everyone.

Bill N.Y.

I have heard many answers about the lifetime of a desiccant cartridge. I changed mine in 2003 when a new one came attached to the rebuilt air dryer assy that replaced the one leaving me stranded.

And I just changed that one at my pre-trip maintenance this week - just on principle. I have never seen water at my drain valves.

How often should a desiccant cartridge be changed?

quote:

Originally posted by garryp:
How often should a desiccant cartridge be changed?

It's all over the map on that one. It depends on several conditions.
1)Climate running
2)Mileage
3)Air Compressor health
4)Money
5)Your Mechanical ability

If you use your coach everyday then change it out before 500,000 miles. I've seen some trucks go 1,000,000 (1 million) miles on an air-dryer. If you have less then 500,000 miles then look at the 10yrs or less rule.

Penske's maintenance department follows the 600,000 miles changeout rule. I still have to chase them and bypass air dryers on a regular basis.

If you run your hands underneath the blow off port and get oily residue then it might be time to replace the air compressor and dryer.

Myself, I don't have an airdryer on the coach. If I did, I would make sure that it had a blow off in the head of the compressor and keep a spare connector to join the 2 lines together.

I wouldn't change mine until I had too. Money doesn't grow on trees and I do have the mechanical ability to "get by" in most worst case scenarios.

Back to Ray's issue--If your system is going down from 120 to 105 in 8-10 minutes while driving, that is fairly normal, when mine is working both the cruise and heater, it can go down to low pressure cut in (mine is a little higher then yours at 110)in less then 5 minutes.

Don't forget, there are two venturi devices in your coach if you have a vacuum operated cruise control. One is at the back usually mounted low and to the right side (passenger side) of the engine for the cruise control, the other is mounted in the front generator area to the left (driverside) of the door for the dash A/C+heater.

You mentioned that you already took one off? if that is the case, putting it back on and operating whatever this vacuum device controls, your air pressure may drop faster because you will be using more air for two devices rather then one.

Again, if you stop the engine, and while walking around the coach you hear an intermittent release of air, it is coming from the venturi device that is still there. If you have a vacuum leak anywhere in the vacuum system that the venturi device is connected to, the venturi device will operate continually (and you should hear a continual sound of air leaking) until low air cut off so you will want to check the health of the vacuum lines and devices there also.

I was wondering does the air pressure get on up to 120# right away or does it take a while? Will the pressure stay low until the MH is driven down the road?

If my system is completely empty, it takes between 5 and 10 minutes to fill.

The rest of the time it seems to depend-on ambient temperature changes and *time*.

Sometimes it hold air really well and only takes a few minutes to top-off and then there are times when the coach has been sitting for days and it has lost half it's air.

...and sometimes it sits and looses very little.

I should have asked this question earlier, but, is this about what I would call full air brakes(if there is such a thing) or air over hydraulic as in my Breakaway?
Jim