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Old 10-23-2016, 11:00 AM   #1
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Parking Brake Plunger Air Leak

Earlier this year my MH was experiencing an increasing number of parking brake alarms and related leveler problems. Searched some old threads here on this subject and then installed a new Nason parking brake switch. Problem solved! Fast forward eight months and I am now hearing an air leak at the parking brake plunger when the parking brake is engaged. I can rotate the plunger knob and reduce the leak noise, but not completely. I'm losing about half the air pressure overnight. I searched again more threads on this problem and I'm a little confused. If I understand it right there is in addition to the parking brake "switch" a parking brake "valve". Having previously replaced the "switch" is it reasonable to assume I now need to replace the "valve"? Thanks for your advise.
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Old 10-23-2016, 11:11 AM   #2
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The parking brake "relay valve" should be between the brake chambers, above the rear axle.

Your new switch may be failing.

To check, you would need to remove and plug the supply line. Then build up air to see if it leaks out of the valve.

Edit, remove the line, going out to the relay valve, and see if air is coming from it or thru the valve.
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Old 10-23-2016, 01:48 PM   #3
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This is the first time I have ever heard of an air brake system's parking brake valve called a a parking brake plunger. It gives me visions of a brown rubber toilet plunger mounted on a wooden stick

But I digress: This parking brake valve, ID'd for the usually by the large square yellow plastic knob mounted to the shaft, can leak internal air from around the shaft and is usually audible to the operator. Replacement used to be the only recommended fix.

The relay valve is on the service brake side of the system and should not affect the parking brake system, AFAIK.

Semi-trucks often have a quick-release valve mounted in-line between the parking brake release valve and the spring brake cans. If you are near a semi and hear the loud pop/swoosh when the driver is parking, that is the air exhausting from the quick-release valve as the spring brakes are allowed to expand and set the brakes. Sounds similar to our air drier's popping off.
I guess the manufacturers did not want Class A's with diesels and air brakes to be any noisier than they are already when parking in a campground?

In the distant past I have had to change out the parking brake valve twice to get one that did not leak.

Good luck
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Old 10-23-2016, 02:45 PM   #4
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What you are calling a "quick release" is a relay valve.

Relay valves move the large volumes of air, needed for stopping, to points closer to the chambers. Without them, all air would need to run to the drivers area and back. That would cause delays in application.

If there was no relay valve, when applying the spring brakes, all of the air would have to exhaust out under the dash.

The workings of the rear dual chambers is complex. There are inversion valves and anti-componding valves, all working together, to make a safe system.
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Old 10-23-2016, 06:15 PM   #5
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Sorry I didn't know the correct name for the parking brake valve. I guess plunger was the first thing that came to mind. Mine does not have the large square yellow plastic knob. Instead, it has a round metal disc mounted to the shaft, but I'm sure it's the parking brake. Maybe I should remove the parking brake valve before I ask more questions, but as I recall from the last time I removed it, there is a valve at the end of the shaft. Connected to it were air lines and a switch with electric wire leads. The switch with the electric wire leads is what I call the Nason switch that I replaced eight months ago. Am I hearing that the Nason switch may have failed and should be replaced again or does the valve it is connected to needs to be replaced?
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Old 10-23-2016, 06:18 PM   #6
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The switchs can fail and the valve itself can leak. The only way to find out is get to it and spray some soppy water on it, looking for a leak.
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Old 10-23-2016, 06:42 PM   #7
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Will do. Thanks to both of you.
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Old 10-23-2016, 08:13 PM   #8
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when you remove and replace the valve.... and all fittings are tight.... take some soap/water or Windex and spray the connections to check for leaks... while your under the dash... if you have air lines running to your air gauges take a minute and check those.. and than if the air lines on the brake pedal are visible... do the same for them....

The next time your under the coach check those connections as well... its not uncommon to find all kinds of small leaks... the more leaks you find and fix the less often your air compressor will cycle, and the longer the drier cartridge will last in your air drier...
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Old 10-24-2016, 09:00 PM   #9
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Thanks jelag. Good ideas.
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