1. #1
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    Default Parking brake leaking air while driving. HME chassis Luverne

    Hi, long time reader. Seldom post.

    Okay brothers, here is our situation. We have this 1994 HME chassised Luverne engine. It has disc brakes all around. When the guys go on a call they run thru town hitting the brakes and releasing them. Hitting and releasing them. Each intersection, each car in the way. All the while the officer is hitting the air horn. Toot toot!

    What happens is the rear brakes overheat.

    What I believe is happening is the air pressure is dropping. The compressor unable to keep up with the demands. We do not have a shore line for air so I cannot say what the starting pressure is. It could be 70 psi or could be the full 130 psi.

    What appears to happen is as the pressure decreases so does the air pressure in the spring brake system. When the air pressure throughout gets down to about 80 psi the brake is applied. As the pressure declines it is applied more.

    Is this the correct way it is to act on a 1994 Fire Engine???

    I would expect there to be a check valve that would maintain the highest pressure within the parking brake. Then if the system pressure gets below a set point it would allow the spring brake to activate in some way.

    Any ideas?
    prepare for the worst<br />Hope for the best

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    I guess the general idea I want to know, is this: How low can you run a truck on air and safely operate it? Shouldn't it operate properly at all pressures above the alarm level (about 70 psi)?
    prepare for the worst<br />Hope for the best

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    Wow! (sarcasm) This board is busy and helpful!
    prepare for the worst<br />Hope for the best

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    is the low air alarm going off ? Have it calibrated.And check the system for leaks. In the mean time buy a small jockey pump
    ?

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    Not a mechanic, but I would agree with yokel. A couple of other things to look at: do you have a jake-brake or something similar? That could reduce the need for the disk brakes and help keep them cool. Also, how often does the rig run? You might check the compressor as well, it might not be sized properly, or may be old and not able to keep up any more.

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    Default Dragging Brake

    Your truck is supposed to have a pressure protection valve that shuts off the air horn when the pressure gets down to 95 psi. The parking brakes release at 60 psi, so there would be enough pressure to keep them from dragging. This valve is required by the NFPA1901 std.

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    Quote Originally Posted by donethat View Post
    Your truck is supposed to have a pressure protection valve that shuts off the air horn when the pressure gets down to 95 psi. The parking brakes release at 60 psi, so there would be enough pressure to keep them from dragging. This valve is required by the NFPA1901 std.
    Not sure if the NFPA1901 Standard that was current for the 1994 vintage apparatus that is being discussed had the pressure protection valve.

    - Tell officer to stay off horn until the problem is resolved. Make it Crystal Clear brakes more important than NOISE!!!
    - Try an experiment where you boost the air system up to operational and see if with normal driving you have the issue. Is it capable of an external air input?
    - See if the truck has the pressure protection valve or not. If not install.
    - See about an additional air reserve tank being added to the air system.
    - See about hooking external air to the truck.
    - See about cost of booster or even small air compressor that can be disconnected prior to departure.

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    Assuming that you have had a mechanic check the brake system, however if not then: 1. Park the engine and securely chock the wheels on a fairly level surface. 2. Release the parking brake and check for leaking brake cans. 3. Try backing up slowly and make repeated applications of the brakes (3 to 5) to bring the automatic slack adjusters into even adjustment. 4. Re-chock the wheels and measure the throw of all brake operating rods by measuring with brake realesed, mark the rod at the can opening, apply the brake foot pedal and remeasure the movement. Compare the throw with the manufacturers specification. Improper front adjustment can cause excessive brake application on the rear. Braking force should be about 40 to 45% on the front and 55 to 60% on the rear (engineered by force and brake surface area). Inadequate braking on the front puts excessive strain on the rear causing overheating. "Cowboy" operating (full throttle & then heavy braking) should be avoided. Have the driver try to get smooth by anticipating openings and traffic movement rather than excessive acceleration and heavy braking. Lastly there should be a "Pump-down check" performed where the chocked engine has the parking brake released and the operator pumps down the brakes until the brake release applies the brakes, the pressure noted. Then the build-up time in minutes should be compared with the manufacturers spec to check the condition of the air compressor.
    Last edited by kuh shise; 05-10-2013 at 07:16 PM.

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    Default Pressure Protection Valve

    I knew I kept those old NFPA stds for a reason. NFPA 1901-1991 std. is what your 1994 chassis should have been built too. Section 3-4.1.3 c) pressure protection valve to prevent the use of air horns and other air operated accessories when the air system pressure drops to 80 psi.

    Easy way to check is set parking brake in neutral and with the engine running, pump the service brakes until the pressure gets down to around 100 psi. Then blow the air horn until the pressure drops to 80 psi. The air horn should shut off and the pressure should not drop lower. 80 psi is enough to fully release the parking brake and keep them from dragging.

    If your chassis doesn't have, need to call the chassis manufactuer and have them pay to install.
    Last edited by donethat; 05-21-2013 at 10:33 AM.

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