Air Use

[Dick Simonis]

I keep getting more surprises with this Marquis.

The latest one first happened when we were maneuvering into our site at Happy Trails Thursday.  After a lot of turning, starting backing and etc. I was 2/3 of the way in the an alarm went off...low air pressure.  Previously the first time I raised the tag I noticed the air pressure dropped about 40 psi and the coach was low in the back.  This took quite awhile to remedy.

It appears that air brakes take a LOT of air when constantly in use and replenishes slowly.  I needed to stop, go the high idle and wait for a couple of minutes....seemed like forever before the pressure was back up.  I suspect the coach was also lowering in ride height so there was a concern about tire clearance.   I did have the tag up with the pressure back to 120 before starting the maneuver.

Than when we go home, same thing happened baking into the Beaver barn where i was using the brakes almost constantly trying to follow instruction.

I hope this is fairly normal and not a symptom.

[Joel Weiss]

Even though our MH's are very different, my guess is that you have some component with a slow leak that is causing your system to be so slow to pressurize.  I've never seen an air brake vehicle in which it was necessary to pull over to get the pressure back up.  I happened to have just replaced the parking brake valve on my MH because it had begun to stick with the brake on.  I have definitely noticed that my system now airs up faster in the morning and holds its pressure better over several days.  I'm pretty sure that the valve had been leaking long before it started sticking.  You may well have something similar in your system that is leaking but at a rate too slow for you to hear a "hiss".

[Dick Simonis]

I wondered about that but the system holds air well when off, it's just the compressor doesn't seem to keep up when just idling and using the brakes a lot.  I step on the brakes and hold the pedal down the pressure drops a bit but than holds steady so there is no apparent leak just a lot of consumption.

[Edward Buker]

At idle the compressor builds air slowly. You have an extra set of brakes on the tag I would guess which can add to the air load. If you are braking and hold the brake fairly steady and not increase and release the brake on and off you will lose less air. If in doubt have it checked out.

Later Ed

[Gerald Farris]

Dick,
The condition that you described is normal if you are repeatedly applying and releasing the brakes on an air brake equipped coach.

Gerald

[Lee Welbanks]

You could have a brake diaphragm leaking, air up the coach to compressor cut-off, engine off, release the parking brake, apply the foot pedal hard and hold it there and see if you air leaks off. You should not loose very much air pressure with one full on hard brake application. If your air pressure drops you have a leak in the service side, valve, hose, brake can diaphragm.

[Dick Simonis]

Thanks for the replies.  I did the brake test thing and the pressure dropped ~10 psi and held so that appears to be OK.  I suspect Gerald is correct in that this is a normal thing when using the brakes a lot in a short period of time.  Make sense if it drops 10 psi per application and I've used them 10 to 15 time in a couple of minutes the pressure can drop below minimum.  Reckon I need to be more aware of this issue when doing a lot of maneuvering.

[Joel Ashley]

I agree that sometimes one has to use the brake a lot when micro-maneuvering into relatively tight spots, especially backing.  But 10 to 15 times in a couple of minutes is quite a few.  That said, I would expect the engine's compressor to kick in before the low pressure alarm sounds off, which on my Monterey is in the neighborhood of 65 psi I think.

Although I agree Dick's repeated tapping of the pedal is the cause of the system pressure loss, the fact that it isn't recovering better is bothersome regardless.  I think the parameters for the compressor are cut-in at around 105 psi and cut-out between 120 and 135 psi, 120 is more common.   I presume he's flipping the switch to high idle when parked and trying to recover pressure with his foot off the pedal, parking brake applied.  It shouldn't take that long to get from 105 to 120.  And if the compressor cuts in at 105 while maneuvering, it should at least counter his braking and not let things fall clear down to the alarm level.  Might he need his air governor checked just in case repetitive braking isn't the only causative agent?

Dick, have you or BCS purged the air tanks lately?  You know,  completely drained the dry and wet sides of each via the gang of 4 brass petcocks?  If any moisture is noted when that is done, I'd consider whether the air dryer/dessicant cartridge needs attention, although I'm not savvy enough to say a problem there would actually slow your progress to 120 psi.

Joel

[LarryNCarolynShirk]

Dick,

I experience the same drop in air pressure when maneuvering.  My solution is shift to neutral and run the idle up to 1100 RPM for a couple of minutes, and the pressure will build up faster.  If you loose 10 PSI each time you hit the brakes, 10 times and you are down 100 PSI ad locked.  I think your experience is normal.

Larry

[Jim Houghton]

One other issue that can cause excessive air use in an airbrake system may be improperly adjusted brakes. If the brakes are too loose. This can result in higher air consumption with each application. This is an issue on OTR trucks.

[Lee Welbanks]

With this coach I don't think it uses much air when applying the brakes, it has disc brakes except for the tag. When parking I usually don't have to use the throttle at all because the coach will move at idle, have to use foot on the brake at all times.

[Gerald Farris]

Dick,
Like I said above, the condition that you described is normal for your coach. The SMC era Marquis with ADB brakes uses air much faster than the engine air compressor can replace it if you apply and release the brakes 15 times when parking with the engine at idle. Do the math, if your air tank pressure drops 10 PSI with each brake application, you are using a lot of air, and an idling C 12 air compressor will not stay up with that consumption rate if it is over a very short time.

Gerald

[Andy Clark]

Another thing to check is the air purge valve located in the base of the dessicant canister holder. Mine has begun leaking and the compressor has to work harder as a result. I lose more air pressure now when braking at low speeds than I used to, but as yet do not run out of pressure. The air escapes from a quarter-sized hole in the base of the cannister holder and can mostly be heard hissing after the engine is shut off.
Just something else to check. My service agency sez that these valves do fail and that there is a kit that can be installed to fix the leak. The coach goes in this week for the fix.
HTH
Andy