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Old 12-06-2014, 12:03 AM   #1
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On board air compressor problem

Hi all,
I am having a pressure issue with my on board air compressor and hoping for some advice. I have a 2000 Allegro Bus with a 300 Cat. I hook up to my compressor by starting engine and letting engine pressure get built up. However, when i hook the compressor up I can only get about 95 lbs which won't fill my 110 lb tires. I don't know if it is related, but my air brakes suck also. The freightliner guy adjusted the slack adjusters and said the brakes are fine and my shoes are in excellent shape. . However, shoving my foot to the floor and hitting the retarder to stop coming down an exit ramp at 40mph is not confidence building.......I have managed to get 5,000 miles like this but having minor panic attacks when someone pulls out in front of me is getting old. Any thoughts on either and could these problems be related?

Thank you so much for any thoughts.


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Old 12-06-2014, 12:56 AM   #2
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On board air compressor problem

Mike.... I can't address the brakes, if FL says all the parts are in order they ought to work properly. Did FL also check and verify that your air system is hitting the right numbers?

On the air, nominally the system should build up to the cutoff at about 120psi, then bleed down through usage to about 90psi, where the valve opens again and allows the pressure to build again. 90 to 120psi-- that's your operating window. If you need to air up a 110 lb tire, your window is even smaller. If you 're only getting 95psi max then something needs to be repaired or adjusted.

Some folks have worked up a Rube Goldberg kluge that bleeds the system down while you're airing up to force the compressor to kick in. Of course, that doesn't change the fact that you must still have over 110psi or you will be bleeding the tire down, and it also requires that you run your main engine just to air your tires. My preference was to go buy a small 110v powered 150psi compressor. Does the job and useful for other things , too.

Good Luck!

John & Diane, Fulltimers. RVM103 NHSO
On the road since June '12 with Lincoln, the guard cat.
2002 Dutch Star 40, Freightliner, Cat 3126, 2004 Element
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Old 12-06-2014, 07:10 AM   #3
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Actually, 95-100 is about what you can get out of the onboard system without some xtra effort. Because the compressor doesn't cut on until around 90 psi, the pressure falls off as you pump air into the tire. You then have to stop adding air to the tire and pump the brakes (or otherwise bleed air below 90 psi) to trigger the compressor to come on and bring it back up to 125, then add a bit more air until the pressure drops again. Repeat as needed to get 110 psi.

As for the brake problem, I doubt if it is related. The brakes only need about 65-70 psi to operate well. Since you have had the brakes checked and they are OK, maybe you are simply expecting too much? That's a lot of vehicle you are stopping, so it takes time (distance) under the best of circumstances. You should have another truck brake shop test drive the coach and verify that braking is fully functional. You might have a bad treadle valve (the thing the brake pedal operates to apply air.
Gary Brinck
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Home is in the Ocala Nat'l Forest near Ocala, FL
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Old 12-06-2014, 07:41 AM   #4
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Another consideration would be the brake shoes are "glazed" and need to be sanded down or burnished. Check out this site and see if it helps at all.

RV Tech Library - Burnishing Squeeling Brakes

There are many post in this forum about these issues. Just google (upper right button on this page) to read many issues. Check out this thread:

High performance shoes?

As a word of caution. On my coach weighing in at 30,000+- stopping is not a big problem. I know I can send my copilot tumbling down the isle if she gets up to go to the bathroom and I have to stop suddenly! However, the stopping is not like in a car. It does take longer. I'm concerned that maybe your coach does have a problem. Maybe either the front or rear brakes are not functioning.
BTW, the slack adjusters are self adjusting. They can be adjusted manually, but a properly maintained system they should never need to be adjusted.
Good Luck, Be Safe and Above All, Don't Forget To Have Fun
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Old 12-06-2014, 07:51 AM   #5
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If this unit had a brake job in the past, it is possable that the brake linings used, are a lower friction material.

Back in the day, our brake shop tried to use the same stuff they use for 10 wheel garbage trucks, on 6 wheel garbage trucks. They didn`t stop well.

Good Luck
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Old 12-06-2014, 08:10 AM   #6
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With enough planning, your retarder will slow the coach enough so that minimal brakes are required using the service brakes. If you are expecting 30,000 pounds of coach to stop like a car then you will be disappointed. I get at least one bonehead a trip that will stop in front of me without warning. It is always exciting. You just have to slow down and plan for it. On the exit ramps you have to start slowing before you get to the ramp.
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Old 12-06-2014, 08:14 AM   #7
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Their has been much discussion here on iRV2 about using the onboard compressor for airing tires, I used that system for a couple of years. To me it seamed to be a Mickey Mouse approach to airing my coach tires. This past month I decided to purchase a 150# pancake compressor from Harbor Freight. It was on sale and I paid just under $100.00 for it. Being portable I can use it around the house and set it in a side compartment when I take a trip.

Don & Bev Morgan Weyauwega WI, 05 Itasca Horizon 40KD, 400 HP Cummins, Delorme GPS LT 40, Toad 07 Saturn Vue AWD, Air Force One, TST 510 TPMS, Mayor of Weyauwega 2007 - 2013, Waupaca Co Board Supervisor 2010 - 2014
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Old 12-06-2014, 09:05 AM   #8
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Forget the air...focus on brakes.

They should be ok on lower air but what do I know.

Take it to a service place that will road test it and see what they say.

If the place that adjusted them did this and you are still concerned then check with another dealer or do a timed stop like 60 to 0 with hard braking and follow up with manufacturer to see if it is correct.

It may be just fine and you need to learn how to drive it or it could be broken and needing repair.

Brakes are top priority for operation so stay focused on those.

Your pressure control system needs to be adjusted or repaired as it seems low.
Tony & Lori
1989 Country Coach Savannah SE
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Old 12-06-2014, 08:14 PM   #9
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Your coach should be similar to mine since it's a Freightliner chassis. I have 105000 miles, and the brake shoes still have 50% of the lining left.

As for stopping, I too can send my copilot tumbling down the isle if she gets up to go to the bathroom and I have to stop suddenly! I was recently cut off in traffic, and the coach stopped so quickly I inspected the tow bar to be sure it was not bent. I've always been pleased with how well the coach will stop.

I too suggest another shop, or at least have another motorhome owner (that has air brakes) drive your rig to get another opinion.

Fred & Vicki
St. Augustine, Fl.
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Old 12-07-2014, 07:00 AM   #10
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I drove & owned a trucking company for 35 years, your brake problem is propobly a bad foot valve, the ports & o-rings get old & dirty & when you apply pressure (pressing down with your foot) it feels like your stepping on a plum.
Have your foot valve replaced with a rebuilt one.
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Old 12-07-2014, 11:54 AM   #11
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Doing an Air Brake Pre-Trip test can demonstrate how your air compressor is doing.

Also, I understand you can set up your automatic slack adjusts on your rig while doing the DOT Pre-Trip Air Brake Test. The automatic adjustment is done during the brake pump down process.


This system is operated by air from a compressor off the engine. Normal air pressure is between 80 and 120 psi.

When the system falls below approximately 60 psi, the brakes are automatically locked by springs and the bus becomes immobile.

TEST 1: Parking brake test.

Start the bus and let the air pressure build to at least 90 psi. Step on the brake pedal and put the bus into gear. Slowly take your foot off the brake and gently press the accelerator.

The bus should not move.

Release the parking brake and use wheel chocks.

TEST 2: Applied pressure test. (With parking brake released.)

Turn the bus off. Step on the brake and hold the pedal down for one minute.

There should be no more than 3 psi of air loss.

Listen for any possible leaks.

TEST 3: Service brake warning system test.

Turn the key to the on position without starting the bus. Pump the brakes until the pressure falls to 60 psi.

At this point, the warning light and buzzer should come on.

TEST 4: Emergency brake test.

Push in the yellow brake knob. Pump the brakes down until the knob pops out.

This should happen at or before 35 psi.

Gently press the accelerator, the bus should not move.

When operating an air brake bus, periodically monitor the air-pressure gauge. If there is a problem with any of the tests, DO NOT use the bus and contact dispatch."
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Old 12-07-2014, 01:22 PM   #12
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Mike....a LOT of people over use their retarder, engine brake, exhaust brake because they feel the need to save those expensive service brakes. The brake assist, no matter what type you have should be used for freeway and off ramp conditions. Turn it off while around town and exercise your brakes.

All of that assisted brake use will glaze over your brakes because they are being only lightly used., basically polishing the surface

I know you say that you really have to step on the brakes on an off ramp. Do you mean that you really have to step on them compared to your car or they just don't work?

For years on my Monaco Diplomat, I felt like you and didn't have a lot of faith in their ability in a panic stop. Not something you test in a loaded motor home. It always took more foot power to stop than my car or truck.

On a trip one day, we were cut off and I had to make a panic stop. I thought for sure that I was going to nail the guy based on past stopping experience. I stood on the brakes harder than I had in the past and was amazed when I almost put everything through the windshield in coach.

My point, you may be amazed at what your brakes can do if you REALLY get on them. If you have and it doesn't stop......you may have other issues.

Lastly....you'll never get your air system to efficiently fill your tires. There just isn't enough continuous pressure developed at one time. Like many of us, just buy a small electric compressor. You don't need the engine on to fill your tires, allowing you to use whenever you want without having to run the engine and fill the tanks.

Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab
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