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BRAKE CHAMBER AIR LEAK.
First some background. All our coaches have air brakes and a pre-trip DOT type brake check is recommended. The reason for this is to verify all portions of the air brake systems are functioning. Part of the test is to verify that when the emergency or parking brake is released that there is no leakage. The standards allow for some leakage, but the reality is our brake systems are to be leak free.
When we release our parking brakes we are allowing air pressure to enter the parking or emergency brake side of the combination brake chambers used on our tag and drive axles. The air pressure compresses a large spring that is used to hold the brakes “on” when parked. When we release the brakes air will enter the parking brake chamber, compress the spring and then as long as pressure is maintained the brakes remain released. If there is a leak the air compressor will have to run to make up for the lost pressure. If there is a leak the brake pressure gauges will cycle up and down frequently.
The reason this is being mentioned is Peter noticed a fluctuation in his pressure gauges after having new brake chambers for his Knorr-Bremse brake system. This was not normal and the frequency of the cycling of his air compressor indicated he had a leak. The leak was actually leaks in both new brake chambers. He had two more new ones installed at a second Prevost service center. The two that were replaced had only 2200 miles on them. The new ones had no miles and they leaked. A third pair was checked, new from stock and they leaked as well.
A reasonable question is “what’s the big deal?” The simple answer is they should not leak. The not so simple answer is if they leak when brand new, how long will they last before a pre-trip brake check shows the leak rate to exceed the limits. As a point of reference both Peter and I have had our coaches parked in our shops with the parking brake released. Both coaches retained all air pressure for more than a month. Not some, but all air pressure. The same should be expected of all new brake chambers.
The down side apart from a likely increasing leak rate is the air has to go somewhere, and where it goes is to the service brake side of the brake chamber in this case. If that air cannot escape it will eventually pressurize the service brake sufficiently to apply the brakes or at least get them to drag. That’s the kind of thing that starts fires.
But there is good news, we think. If air gets into the service brake side of the brake chamber it appears the increased pressure begins to cause air to leak from the ABS modulator valves. That should prevent a dragging service brake, we think. I keep repeating we think because that is the case on Peter’s coach, but other brand modulators may not respond the same way.
We are still attempting to get the big picture on this issue. Prevost is certainly aware of the problem now, but we do not know if they will react, issue a service bulleting, a recall or what they will do. We will follow up and perhaps we can get some guidance on this issue. A call was placed to Bendix (who is owned by Knorr-Bremse) to find out their position on this problem. So far we have been unsuccessful at speaking to someone at Bendix for their advice.
We do not know if this is an isolated problem with a single batch of brake chambers or if it is a widespread issue. Right now the leak rate is not large enough to be a safety problem, but being that there is a leak when there should be none we have no clue if the problem gets worse very fast or if it is a minor issue. We do know at least two things for sure. All the brake chambers (6) that Peter had installed or tested leaked. The seond thing we know for sure is as long as the coach has Wabco ABS modulators the design allows pressure on the service brake side of the chambers to be relieved through the exhaust port on the ABS modualtor. We strongly suspect if a coach uses the Knorr-Bremse brake chambers and they leak, and there is no ABS modulator in the service brake side such as is the case with tag axles the service brakes can drag or lock up.PLEASE NOTE THAT IF YOUR KNORR BREMSE BRAKE CHAMBERS LEAK ON AN AXLE WITHOUT ABS MODULATORS YOU COULD HAVE A PROBLEM.
It appears this is a serious issue and we suggest until we have better information of answers that you be especially alert to excessive cycling of the air compressor. We suggest if you suspect the cycling is more than it should be that you do frequent temperature checks of your wheels and hubs at every stop.
Peter and Jeff may wish to add to this discussion. In the meantime we will all press for someone to address our concerns.
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3 Replies
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If you want to check your coach for leakage on your brake chambers when the parking brake is released, remove the service brake line from the brake chamber (the rubber hose, not the nylon line), hold your finger over the hole on the brake chamber, spray some soapy water around your finger, and if you see bubbles air is leaking from the emergency brake side of the chamber through to the service brake side and if you do not have an ABS modulator valve in the service brake line air entering the service brake side will eventually apply your brakes.
To do this you need to get under your coach. If you do make sure it is fully supported at the chassis support points.
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The bottom line according to Bendix tech support is the brake chambers should be leak free. However for new chambers Knorr Bremse has an allowable leak rate which is quite low and specific to each size brake chamber. I will post the correct information when I receive it, but the tech said it will be expressed as something like a 1″ bubble in 3 seconds. Anything in that range is minute.
Our concern about pressurizing the service brakes was unfounded. That is good news. On all our brake systems, if there is a leak to the service side it will exhaust to atmosphere either at the ABS modulator valve or the relay valve because they are open to atmoshere under normal conditions. If you do have a dragging brake it would have to be due to a system failure other than a leak in the seal between the emergency brake and service brake sides of the spring brakes used on our tag and drive axles.
As more infomation becomes available I will post it.
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A follow up. As Peter posted on another thread he has replaced the second set of brake chambers that were installed on his coach with a third set, this time apparently from a different batch which came from the Prevost NJ service center.
When bench tested these did not appear to leak, something confirmed by Peter. So it is clear that some brake chambers do leak. The allowable leak rate has not been defined yet by Bendix but after bench testing at Jeff’s it appears the brake chambers that were supplied by Mira Loma and Nashville, probably from the same production batch did have a leak greater than the ones now installed.
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