understanding air on diesel motorhome

[kokofriend]

OK, I am still a diesel newby. I have 2 gauges that are front air and rear air. I pump them up to 120psi on start up. While driving, the front air gauge drops to 100 and then gets pumped up. The rear air doesn't drop, but sometimes goes up to near the 150. When I park and camp and then get ready to leave, the rear air is sometimes already up. Do I have a leak in the front air? Are the air bags that raise and lower the height of the motorhome for travel different from what the gauges monitor? I have a 2004 FL chassis.

[Martind4]

Here is an excerpt from one of my previous posts explaining how the air system on a Freightliner can be used to pump up tires, etc. It also explains how the two tanks work together.

I just attended Camp Freightliner and probably learned just enough to be dangerous. Here's what I remember about using the air manifold as a source to inflate tires. The manifold is fed from the secondary air tank which is also used for front brakes and other accessories (air horn, etc)

The primary tank is used exclusively for rear brakes. Both tanks are connected together and both are protected by Pressure Protection Valves. These valves are used to protect one tank from another by closing automatically at a preset pressure, should a reservoir failure occur. Loss of air from one tank should not drain other tanks if the valve operates correctly

That means your tire filling process will drain the secondary tank and the primary tank still has enough pressure to not kick in the compressor. It was recommended that we add a pressure gauge between the end of the air hose and the air chuck. When you're filling tires and the hose pressure drops below the desired level, have the DW pump the brakes enough to lower the primary tank pressure and kick in the compressor. Resume filling the tire when the hose pressure is high enough.

This makes it a 2 person job but it eliminates finding space for another compressor. I strongly recommend Camp Freightliner!

Note: The engine must be running for all of this to work.

[Skip426]

Leak in the front air should be audible with the engine off.With the air pressure up and the engine off. Block the wheels, release the park brake, have a helper apply and hold the brakes hard, listen at each wheel for air leaking. That will eliminate the individual lines from the foot valve to the brake pots and the pots themselves. If you notice a drop in front tank air while sitting or driving without brake application, you will have to get close to the tanks to check the manual water drain valves for leaks, and the supply line from the tanks to the foot valve.CAUTION THIS AREA IS UNDER CONSTANT HIGH PRESSURE: USE HAND & EYE PROTECTION>!!! On my coach this is done with the gen set moved forward and by crawling in behind to get close. These 3 valves; 1 on each tank and1 on a splitter valve; have cables attached for maintenance, and should be run to some where that can be accessed easily; mine were not. Manual draining of the tanks and checking for water, misting out with the air , is an indicator of the performance of the Auto Air Dryer. Excess water, more than 3 to 5 sec, of mist when draining indicates a need for dryer service or repair. If no air leaking there, you should be ok. The air leveling system may effect your gauges while driving, I haven't noticed if mine does. I think dumping the air levelers on my coach drops both gauges equally. Downloading a F/L chassis manual from their website may answer more of your questions, and joining ACCESS FREIGHTLINER, will get you a lot more. Good Luck , and keep asking questons , the more we know the safer we are .

[kokofriend]

Thanks, I am thinking of going to Camp Freightliner, too. I am not physically able to crawl under and check, but will see about getting it checked by the freightliner dealer.

Barb

[Skip426]

On thinking about your questions overnight,MHO, I think that in the absence of any audible leak, you may be paying more attention to the gauge readings than the system requires.
If your low air warning buzzer activates properly, and your compressor will build pressure in the system at 1000 rpm idle from 60 to 90 psi. in under 2 min. Your system passes the testing that truckers with air brake systems should do every morning. The operation of the auto slack adjusters, should be checked and greased at least anually, more often if your mileage per year is high.
After start up and initial pressure build, I check the pressure about once an hour, more often on hilly roads and every time before cresting a known long down hill. If you don't have good air pressure stop at the crest of the hill, check for leaks and let the pressure build, before starting downhill. Safe Travels.

[MaverickBBD]

I know from the outside it looks like you only have two tanks but in actuality you actually have five tanks, a wet tank (easily identifiable as it has an automatic bleed valve), a primary , a secondary and two ping tanks. The ping tanks collect the air from the air bags that is "squeezed" out as you travel and then takes it back. You "front" air gauge is the pressure of your secondary air tank. In addition to front brakes it also supplies air to your air bags. That possibly is what you are seeing when you go down the road. After you start and air up, do you see a drop in pressure if you are just sitting?

[kokofriend]

Thanks, Tom. I don't see any leak when I am sitting. I will watch it again. Also, I never get a low air warning while driving. I need to get the manual back out.

[J Walker]

An easy test is to see how the pressure is retained after you turn your ignition off. After a day or two I still have nearly full tanks. If yours don't leak down to less than 60 psi in a few hours you don't have a significant leak. The governor on your compressor cycles. It pumps the pressure up to a set level, usually about 130 psi, and then turns off. When the pressure drops to another set level the compressor kicks back on, thus the cycling of pressure high to low. Having one tank read 150 psi and the other less could be a sensor problem giving a false readout. If you really are getting 150 psi in one tank I would be concerned. It could also be a bad gauge. A good shop should be able to narrow down the fault pretty quickly. There is definitely a fault and air pressure with air brakes is something to attend to.

[JCM]

This might help.

[chasfm11]

Barb, if you are loosing pressure on the front system while driving, you definitely have a leak. It probably isn't in the brake part of the system but in one of the connections that go to it. We had the sense line (the thin tube that goes to gauge control) crack and break. The compressor was kicking on every 5 minutes.

If you get both tanks completely full and shut of the engine in a really quiet place, you may be able to hear the leak. Otherwise, the only way to find it is to trace each of the air hoses and put soapy water on the connections. A good truck shop should be able to help you with that. It isn't difficult, just tedious. My guess would be that you have a loose hose clamp somewhere. Those are more common than punctured lines.

Good luck.