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Brake Test Failure. Where to Start the Witchhunt
So while holding brake at full air tank(s) pressure @ 120+PSI, immediately my bottom right pressure gauge labeled “R” (Secondary) proceeded to loose air rapidly. Other gauges labeled “A” (Accesories) & “F” (Primary) dropped @ 5psi and held. This failed brake test.
Question is where to begin finding leak ? Am looking at Air schematic to learn this system but any help is appreciated. Tends to hold full pressure on all 3 gauges for @ 3 hours , then slight drop in pressure over a day or so while parked.
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12 Replies
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If I understood your description of losing brake pressure by holding down the brake pedal the service brakes are in two systems, a primary (rear) and secondary (front).
When one drops that is the system you need to focus on. You do not say how fast it drops, but assuming you can see the gauge move it likely is making noise as it leaks down. Have someone hold brake pressure (engine off from full pressure) and listen for the noise. At the very least you will isolate it to a side, and if loud enough you can likely hear it at a wheel.
If you air up the systems, turn off the key and pressures hold that is nothing but good news because the source of the leak is going to be somewhere between the related brake chamber, its hose and fittings and the brake relay.
Could be a hose, leaky fitting or a failing brake chamber diaphragm, If it leaks down without your foot on the brake it is somewhere from the relay valve to the associated air tank.
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Jon you are good! That is a great description on what to look for. Last year I replaced all my brake pots and rubber hoses.
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I’m assuming Steve has Knorr-Bremse (not Meritor) brakes. The brake chambers are expensive, especially the combos in the rear, but like the cheap (relatively speaking) brake chambers used on the Meritor disc brakes the same parts are going to age, wear and fail. I used to change brake chambers every 5 years, but with the Knorr-Bremse I decided to push it and change them every 10 years, and I have.
The last time I researched the Prevost interval they stated brake chambers need to be replaced every year or 100,000 miles which ever comes first. I’m not sure with our limited use of coaches that is valid for motor homes, but we all need to keep in mind the service brakes have rubber diaphragms and they do age out.
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Leak is coming from ABS valve (Modulator,) it on right rear just inside of tag. Probably should change side too . Meritor website part #A7441011-001 $159.
Changeout looks like real fun. Wondering when air lines to this are disconnected does that lock parking brakes! Wheels are chocked anyway.
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Steve,
When your parking brake is applied there is no air pressure between the relay valve and the brake chamber. Your parking brakes are applied by very heavy springs and when you release them you are using air pressure to compress the spring. You can work on the modulator valve and hoses within the system and there will be no air pressure and the parking brakes will remain applied.
Mike Harper
2004 Marathon XLII -
Appreciate it Jon. Found part at Frreightliner knoxvil
le
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Now that you have isolated it to a part of the system I completely forgot to mention (it is between the relay valve and brake chamber) I wrote an article on doing an ABS system Huff test. It would not have identified the problem or isolated it, but it is a fun test to hear the ABS system at work.
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Where might this article b found
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In the articles section of this site.
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We also did a video of the Huff check, go to Videos of Interest.
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Good video . Will test .
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ABS Modulator Valve is the failed part. Removed item is Wabco Part # 4700 47219500 60 and is 24 volt. Cross reference Meritor part # R955397. Pricing varies from $150 – $390.
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