How fast should your air pressure fall?

[it_mike]

As asked, how fast should your pressure fall off after shutting down?
(another item they failed me for)

[luvrbus]

less than 3 psi in 1 minute with parking brake released



good luck

[Highway Yacht]

As asked, how fast should your pressure fall off after shutting down?
(another item they failed me for)

What type of Inspection and in what state was this in?

[Jriddle]

less than 3 psi in 1 minute with parking brake released


and service brake depressed
John

[it_mike]

What type of Inspection and in what state was this in?

Virginia, standard vehicle inspection

[akroyaleagle]

Here is the DOT mandatory check during drivers test.

AIR BRAKE TEST SINGLE VEHICLE

With ENGINE OFF, AIR PSI at MAXIMUM, wheels chocked, spring brake released,
and key on.

1)  Air Leakage Rate

     a. Watch the air supply gauge for 60 seconds, the air loss should be no
         more than 2 psi
     b. Apply the foot valve fully, Watch the air supply gauge for 60 seconds,
        the air loss should be no more than 3 psi.

2) Air Warning Light

     Apply and release the foot valve until the air warning light comes on, this
     should happen before 60 psi.

3) Spring Brake

     Apply and release the foot valve until the spring brake pops out, this should
     happen between 20 and 45 psi.

4) Air Build Up Rate

    Start the engine to build air pressure, when air pressure reaches 85 psi, time
    build up rate to 100 psi at idle. It should take no more than 45 seconds.

5) Govenor Cut Off

    The govenor should cut off at about 110 to 125 psi.
    You will hear air release from the system.

6) Spring Brake

    a. Remove the chock blocks.
    b. Place the vehicle in drive.
    c. Attempt to pull the vehicle forward, the vehicle should not move.
    d. Place the vehicle back in neutral.

7) Service Brake

     a. Release the spring brake.
     b. Place your foot on the foot valve.
     c. Place the vehicle into drive.
     d. Put the vehicle in motion, when the vehicle reaches the speed of 5 mph
        apply the service brake to stop the vehicle. The vehicle should stop!

[it_mike]

But these aren't spring brakes... 

Also, I expect it takes longer to build air pressure due to the suspension.  I usually plan on 3-5 minutes minimum to get above 100 psi.

BTW, it's been determined that I have a leak in my RH tag air bag and the rear brake chamber.  Working up the parts list now with what's available online, but it's missing sections.  Can anyone look up the rear (drive axle) brake chamber rebuild kit (diaphragm and rubber parts) and the drag link for a 1980 MC9?

Thanks,
Mike

[buswarrior]

The tests needed are the ones done by an inspector/mechanic for inspection purposes, which in some jurisdictions is a whole different bunch of things from fooling an examiner to license a driver.

As noted above, for the air leaks, 3 pounds in one minute, parking brake released, service brake to the floor, engine off.

The air pressure climb is done after the system is full. Fanning down the pressure with the brakes and then time the build-up time.  45 seconds is too short for the regulation, however, that would be a good time for a healthy compressor.

get the tests for your place, and keep fixing it until it passes.

happy coaching!
buswarrior

[bevans6]

Here, the DOT standard for "compressor recovery" is 2 minutes to recover from 85 PSI to 100 PSI, engine at 1,000 rpm (high idle).  As noted, that is after the bus has completely aired up and the suspension has settled out to where it is happy sitting.  the suspension will pull air to level itself if it is not level or it drained down.  The truck with spring brakes that I took my air brake test on took 45 seconds to recover, my bus takes 17 seconds or thereabouts.

Brian

[akroyaleagle]

The tests needed are the ones done by an inspector/mechanic for inspection purposes, which in some jurisdictions is a whole different bunch of things from fooling an examiner to license a driver.

BW and others,

The procedure I quoted is straight from the DOT manual (which doesn't apply to non commercial vehicles but is a good practice to follow).

It is required of all CDL applicants in Alaska and may be checked in commercial vehicle inspections by DOT at any time. When we ran tour buses, I required it by each driver during their initial driver check and every annual re-certification. I have also shown it to anyone interested since we got our coach. I believe it is necessary to understand to ensure the system is healthy, especially in our coaches that are rarely if ever looked at by mechanics.

Maybe I didn't understand the original question. I thought it had to do with how to check the system. It now appears it had to do with an airbag leak.

[it_mike]

No, I appreciate the input Joe.  The only question I had regarding your (DOT's) procedure was whether it would be the same for non-spring brakes. Apparently DD3 brakes have different testing requirements.