Air Leak Mystery

[truk4u]

I have been fighting an air leak battle and have run out of options. Any suggestions (with hands on experience) would be appreciated.

Problem 1 - Overnight the aux, primary and secondary air tanks drop from 120psi to 40 psi, at zero, the tag dumps as designed.

Problem 2 - By filling the aux tank with shop air, it also fills the brake tanks. It's not suppose to do that. This is a bad deal when using the aux compressor, as it has to run a really long time to pump up the aux tank and both air tanks. The rear air fill puts air to all tanks as it should.

Problem 3 - There are no leaks that I can find. I have gone over every fitting, valve, line, gen air bags, step slide, toilet, dump valves, treadle valve, shutters, seat, horns, engine compressor and dryer with the electric stethoscope and no leaks. I can't find the air governor, it's not on the compressor which is barely visible on the 8V and I can only see two lines coming off the compressor, so that I haven't checked. The suspension is tight and no issues there.

According to my vintage Prevost Drawing, there is only one check valve in the entire system and it is a double located up front. My plan is to replace that check valve, but that doesn't solve the leak problem, the air is going somewhere.

The brake tanks should hold for a month with little loss and I could live with aux mystery leak, but the combination of the two is troublesome.

[MangoMike]

Problem 2 - By filling the aux tank with shop air, it also fills the brake tanks. It's not suppose to do that. This is a bad deal when using the aux compressor, as it has to run a really long time to pump up the aux tank and both air tanks. The rear air fill puts air to all tanks as it should.

I know the main tanks will stop filling the aux tanks at a certain psi with some sort of check valve. I wonder if that valve only works one way and when filling the aux tank it will keep dumping air into the mains.

Tacky side comment: You should've kept Big Red.

m

[Joe Cannarozzi]

Follow the 1/4 inch line off the air dryer and that will lead you to the governor.

On every 8-V I have seen it is on the compressor. Sit up between the rear end and tag and look back. If you can remove it from there you have longer arms than me. I pulled the rear floor.

[truk4u]

Joe,

It's probably on top the compressor as you are indicating, it's not on the back and that's why I can't see the governor lines. I think your right, the bedroom floor may have to come out to get to it unless it's remotely mounted (I hope). If I had a leak at the governor fittings, I think I would hear it at the Dryer with the stethoscope.

Mango - Thanks for the closing remark.:(

[Jon Wehrenberg]

Tom,

Something doesn't make sense and maybe I don't have a clear understanding of the air system. I can dump air from a front tank for example and make the aux air pressure go to zero, but air is still retained in the next tank back. I can dump air in that tank, and air is still retained in the next tanks back, etc.

When I air up my aux system at the front Schrader valve it is just the aux, and not the brake systems. This is confirmed because if my brake systems were at zero the compressor would only air up the aux system, and that includes the portion related to the suspension and not only the converter added stuff. I can only air up all three systems from the rear Schrader valve or the engine driven compressor, and until the brakes are at or above 70 to 75 PSI the aux system does not get pressurized.

Where I am headed with this is that a leak at the compressor or dryer should have no impact on the system pressures because of check valves. Additionally if the aux air goes to zero as mine does when I don't keep the compressor running when it is in my garage, my brake systems are both sitting at 120 or more for weeks and weeks.

What you describe is the opposite of all I have been used to and seen. I have never seen all systems fall in pressure simultaneously nor have I ever had my aux air compressor pressurize my brake systems. Either you are smoking some really good stuff, or Brian is laughing his ass off because he modified it to play with your head, instead of going out to feed his moles.

[jimshoen]

Contact your Prevost Tech Rep. They can explain how to test the check valves in your system, and hopefully narrow down your search. I was having a similar problem and ended up finding two air leaks. One at the Discharge Line Safety Valve (replaced it with a new one) and one leak at the large air line leaving the Primary Air Tank on the forward end.

[jimshoen]

Tom,
You must have Big Blu to help in finding leaks.
It is sold at all HVAC suppliers.
See below,
Jim

http://www.refrigtech.com/bigblu.html
BIG BLU
Part #RT100
Micro Leak Detector


FIND LEAKS FAST...Read our
Leak Detection Manual

Part Numbers:
RT100S, Spray Quart
RT100G, 1-Gallon
RT100D, 55-Gallon Drum ONE OF A KIND
BIG BLU needs very little introduction since it is used by nearly 80% of all HVAC/R Technicians throughout the U.S. and Canada.

Bubble leak detection was once considered gross and unreliable. To a certain degree it was true because the dominant makers of leak detector solutions relied on simple mixtures of soap and water
.
BIG BLU is a strong and persistent film forming liquid that has excelled in the field identification of gas leaks. It remains, the only bubble/foam indicator capable of detecting micro gas leakage down to 0.65 ounces per year.

Food Grade, Non-Toxic, Non-Corrosive, Biodegradable and Approved for use on Oxygen systems.

[dreamchasers]

I have been fighting an air leak battle and have run out of options. Any suggestions (with hands on experience) would be appreciated.

Problem 1 - Overnight the aux, primary and secondary air tanks drop from 120psi to 40 psi, at zero, the tag dumps as designed.

Problem 2 - By filling the aux tank with shop air, it also fills the brake tanks. It's not suppose to do that. This is a bad deal when using the aux compressor, as it has to run a really long time to pump up the aux tank and both air tanks. The rear air fill puts air to all tanks as it should.

Problem 3 - There are no leaks that I can find. I have gone over every fitting, valve, line, gen air bags, step slide, toilet, dump valves, treadle valve, shutters, seat, horns, engine compressor and dryer with the electric stethoscope and no leaks. I can't find the air governor, it's not on the compressor which is barely visible on the 8V and I can only see two lines coming off the compressor, so that I haven't checked. The suspension is tight and no issues there.

According to my vintage Prevost Drawing, there is only one check valve in the entire system and it is a double located up front. My plan is to replace that check valve, but that doesn't solve the leak problem, the air is going somewhere.

The brake tanks should hold for a month with little loss and I could live with aux mystery leak, but the combination of the two is troublesome.

Tom,

My 95 has check valves protecting the source for the primary and secondary air tanks (I replaced both). It was hard to detect the check valves on the Prevost pneumatic drawings, but they are there on my system.

I too have been chasing a mystery leak. I have narrowed the search to the primary pressure gauge located on the dash. It seems the 1/8 " fitting on the rear of the gauge was leaking on the tube side. I located the leak by removing the 1/8" tube fitting from the primary cluster, in the same cluster where the shuttle double check valve is mounted and plugged the port to check.

The word patience has taken a new meaning in tracing air leaks!

I have the repair fittings on order.

I will send you a private email with contact info.

Hector

[truk4u]

Jon - Not yet smoking anything, aux filler in steering bay pumps up all tanks! I assume it's pumping up the wet tank, that's the only one without a gauge.

Jim - Thanks, please explain the Discharge Line Safety Valve? I'll try Prevost, but I'm sure the answer will be to take it to a branch.

Hector - I have 185 listed items in the drawing and (1) double check valve. Is it possible that a Converter adds check valves to compliment their systems? There must be other check valves not on the drawing. Example; If there wasn't a check valve at the rear fill going to the wet tank, air would come back out when pressurized.

Thanks for all the suggestions guy's, the beat goes on..