searching for an air leak(s)

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searching for an air leak(s)

September 12, 2014, 10:14:13 am

I have a small air leak in my air brake system. The system loses all air pressure overnight regardless of the outside air temperature. I think the leak is in the brake system because both pressure gauges go down together and they leak all the way to zero. If I had an air leak in some system other than the brakes I would probably have one gauge going down and the other staying up. Also if I had an air leak somewhere other than the brakes it would probably stop leaking when it got down to 60 psi. The air bag suspension stays up even after the air gauges drop to zero. My question is what are the most likely air leak candidates in the brake system? The leak is probably too small to hear so I'll search for it with a spray bottle of soap suds. I'll start with all the hoses and fittings I can get to easily under the coach, the air brake switch on the dashboard, the treadle, the electric compressor and its fittings and valves. What other places should I look? Is there a check valve that can drain the entire system? I suspect the check valves on the front "dry" tanks would only cause one gauge to go down, so I doubt it's one of those check valves. I overhauled my Bendix AD4 air dryer a year ago and replaced the check valve inside it, and the popoff valve in the bottom. I know this isn't a huge air leak but it's been bothering me for a while so I thought I'd search for it. It's probably something that's easily repaired if I can just find the leak.

Re: searching for an air leak(s)

Reply #1 – September 12, 2014, 10:19:39 am

Scott make sure you ck the clear plastic/glass bottle by the compressor. had a crack in mine unseen. DAN

Re: searching for an air leak(s)

Reply #2 – September 12, 2014, 10:27:32 am

I don't have a 12 volt DC compressor. My electric compressor is 120 volt AC and it goes directly to the wet tank so it doesn't have a plastic bottle. It does have at least one check valve inside it and a gate valve with a hose and a tire chuck for inflating tires. I suspect a leak in any of those valves would drain the entire system so I'll inspect them carefully.

Re: searching for an air leak(s)

Reply #3 – September 12, 2014, 10:32:22 am

If you have a leak in the braking system and the air goes to zero//Does that mean your parking brake may not be holding??

Re: searching for an air leak(s)

Reply #4 – September 12, 2014, 10:46:37 am

It's a small leak that goes down to zero slowly overnight. When the engine is running the air gauges go up to 110 psi like normal. The parking brake releases when the air pressure comes up. I just want to find the leak so I can leave my electric air compressor on without it waking me up several times in the middle of the night... (yawn) ...As it is now I put wood blocks in beside the air bags and let the suspension rest on the wood so I can turn the electric compressor off.

Re: searching for an air leak(s)

Reply #5 – September 12, 2014, 11:00:09 am

Quote from: Dan J. Stansel – September 12, 2014, 10:32:22 am

If you have a leak in the braking system and the air goes to zero//Does that mean your parking brake may not be holding??

No, Dan. Exactly the opposite. The parking brake is spring applied air RELEASED. As pressure falls below 30- 50 PSI, the parking brake automatically applies, as their is insufficient force to overcome the spring that applies the brake.

Re: searching for an air leak(s)

Reply #6 – September 12, 2014, 11:46:03 am

You may have bad check valves that are leaking back to wet tank and the wet tank leaking back to air dryer and you would not hear that probably. The foot throttle is possibly leaking but again that should only affect one tank. There is always the possibility that numerous leaks are happening, but with me having gone thru a similar situation and finding bad check valves I always think of them first. I suggest you dump the air totally and take these apart and see. They are simple units to clean up and unless the rubber seal is gone you can get by with just clean up for now to at least rule them out. Just my thought.
JohnH

Re: searching for an air leak(s)

Reply #7 – September 12, 2014, 11:51:54 am

I can't answer to the possible location with the symptoms you described, but I can share where I have had some. One was the compressor check valve you mentioned and the other was the drain valves on the two forward tanks. Another place was the thermostatic valve on the passengers side engine head (thermostat housing). Loosing all pressure overnight is not all that small of a leak. If you park somewhere quiet I would think you might be able hear it.
I solved the mid-night compressor running by installing a heavy duty (15 amp) digital plug in timer in the outlet the compressor is plugged into. Digital so it will keep time even when power is off. John just mentioned the air throttle: the DDEC is all electronic so it wont be that.

Re: searching for an air leak(s)

Reply #8 – September 12, 2014, 11:58:47 am

I was thinking about our set up and did not relate it too Scott's coach, sorry.
JohnH

Re: searching for an air leak(s)

Reply #9 – September 12, 2014, 01:32:01 pm

I removed the drain valves in the bottom of the two dry tanks a year ago because one of them was leaking and in over a year of checking them I've never yet found any moisture in the dry tanks. I put plugs in place of the drain valves. I replaced the pneumatic temperature sensor on the back of the passenger side cylinder head last winter. It was hissing noticeably in cold weather. It controls the pneumatic clutch on the radiator fan. I have a rear radiator with a belt driven fan that has a pneumatic clutch on it. I suspect I'll find a leak at the electric compressor or the parking brake button... but I'm not willing to bet a steak dinner on it ...maybe just a burger?

Re: searching for an air leak(s)

Reply #10 – September 12, 2014, 01:38:48 pm

...as for the check valves, exactly which fittings on the tanks have the check valves in them?

Re: searching for an air leak(s)

Reply #11 – September 12, 2014, 02:34:42 pm

After studying the drawing of my air brake system that Foretravel gave me when I was at their factory last time, it looks like the air leak has to be in the wet tank, one of the air compressors (engine or electric), the air dryer, or the tubing / fittings to the dry tanks or the other devices hooked to the wet tank. The leak probably isn't in the parking brake button or the treadle valve because these are on the dry tanks instead of the wet tank... and I think the leak has to be associated with the wet tank... so that's where I'll be with my bottle of soap suds if you need me

Re: searching for an air leak(s)

Reply #12 – September 12, 2014, 03:45:48 pm

Scott, if your gauges are on the dry tanks then the leaks can be on those systems OR leaking back to the wet tank thru those check valves. These valves are on the main tubes coming into each tank and are like a big bouble nut unit (at least that is mine) They seperate between the flats for wrench's and have just a light spring and a rubber diapghram inside.
http://www.meritorwabco.com/MeritorWABCO_document/pb9926.pdf
Look at the top image on page 14.
JohnH

Re: searching for an air leak(s)

Reply #13 – September 12, 2014, 03:52:37 pm

My electric air compressor apparently has a blown head gasket on the front cylinder head and air is leaking from it. The compressor is probably the original and I had already planned to replace it with a 12 volt DC compressor as soon as I have the money and time. I'm told there's a rebuild kit for the pump, which is made by Gast, and there may even be a 12 volt DC motor kit available for it. By the time I get done with all that it would probably be cheaper to buy a whole new compressor.

I tested all the fittings and hoses on the wet tank and nothing is leaking. I could hear air hissing out of the tank though so I sprayed it and found rust holes in the bottom of the wet tank leaking air. The previous owner didn't drive the coach and probably wasn't aware he was supposed to blow the moisture out of the wet tank periodically, especially when running the electric compressor. He apparently wasn't aware that he could put wood blocks in beside the airbags and set the coach down on the blocks so he could turn off the electric compressor. If he had done any of these things there wouldn't have been water accumulating in the wet tank for the past 14 years... (sigh)... So what will a new wet tank cost? I'll call Foretravel on Monday to find out what they suggest. Hopefully it's a generic tank that's available at any diesel truck shop (?) I need to get the tank replaced right away before the bottom of the tank bursts. Fortunately it didn't do so while my ear was up against it listening for the leak.

Re: searching for an air leak(s)

Reply #14 – September 12, 2014, 04:31:43 pm

They most probably are generic but maybe have fittings to match FT design, but you just plug up what you do not need. Has to be certified I guess. Ask any trucking outfit what they do for replacement.
JohnH

http://www.truckspring.com/suspension-parts/air-suspension/air-tanks/eight-half-gallon.aspx
These guys seem to have different ones. Find the size then see if they have them

Re: searching for an air leak(s)

Reply #15 – September 12, 2014, 04:59:02 pm

Looking on the bright side, I don't have to open the drain valve anymore to let moisture out of the bottom of my wet tank.

Re: searching for an air leak(s)

Reply #16 – September 12, 2014, 05:54:16 pm

I think I'm gonna put a check valve on the output side of the electric air compressor until I get ready to overhaul / replace it. I was surprised there wasn't one there; or if it was there I didn't recognize it. Maybe there was a check valve inside the compressor that wore out or something? Anyway, the compressor still works even though it sounds terrible and has a leaking head gasket. It's a GAST 5H which appears to be a super expensive compressor. It's got a 3/4 horsepower 120 volt AC motor on it and a twin cylinder piston compressor that produces up to 4.7 cfm and up to 100 psi... which probably explains why Foretravel didn't use a 12 volt DC compressor. A motor like that would be murder on the batteries.

http://www.gastmfg.com/product_detail.aspx?ProductID=458&ProductTypeID=23

Re: searching for an air leak(s)

Reply #17 – September 13, 2014, 12:09:01 am

Quote from: RRadio – September 12, 2014, 05:54:16 pm

I think I'm gonna put a check valve on the output side of the electric air compressor until I get ready to overhaul / replace it. I was surprised there wasn't one there; or if it was there I didn't recognize it. Maybe there was a check valve inside the compressor that wore out or something?

I've changed my compressor plumbing so I'm trying to remember exactly where the check valve is in the OEM setup. The output line (plastic air brake line) of the compressor feeds into a few fittings with a valve controlling flow to the tire fill hose. IIRC, the check valve is between the valve and the plastic air brake line. You probably didn't recognize it because it looks just like a standard 1/4" hex nipple. It has some plain steel parts inside that rust and mess up the seal.

I once checked out using the same 12 volt compressor that Country Coach put in their new coaches in 2008 - it was about $500. I decided the 120volt unit was fine. I did install a "service kit" on mine back in '07. I had to order it online and I don't remember it costing that much. I got the number from the on-line manual. These compressors are very simple. I would pull the offending head off and make a new gasket from some similar material if the original gasket is unserviceable. Just keep good track of the plates and parts and their orientations.

P.S. I think the new tank needs to be DOT Approved like one of these: http://www.surpluscenter.com/Brands/Hoosier-Tank/6-31-GALLON-MOBILE-AIR-TANK-4-1754.axd

Re: searching for an air leak(s)

Reply #18 – September 13, 2014, 10:03:05 am

It looks like the compressor has check valves inside each cylinder. I ordered the rebuild kit, a GAST K263 for $103.63 from Buy Pumps Now on the internet. I think the GAST 5H compressor sells for about $700!

Re: searching for an air leak(s)

Reply #19 – September 13, 2014, 10:13:51 am

Does your coach have a good drier system installed on the electric compressor? That's why the tank is rusted out. Even if you drain it ever day, it's still going to rust out over time.

I never get moisture out of my "wet tank" because the air drier is doing it's job. But the 12v compressor hardly runs on my unit. It only runs when leveling occurs and pressures are low. My airbags don't leak much and the tanks hold air pretty well and haven't leaked to 0 yet.....well except when the damn front step cylinders leak.....

Re: searching for an air leak(s)

Reply #20 – September 13, 2014, 11:30:44 am

Quote from: Dan J. Stansel – September 12, 2014, 10:32:22 am

If you have a leak in the braking system and the air goes to zero//Does that mean your parking brake may not be holding??

Parking brake is spring applied and requires air pressure to release it. Good question although

Re: searching for an air leak(s)

Reply #21 – September 13, 2014, 01:47:39 pm

The previous owner ran the 120 volt electric air compressor 14 years in Florida and let the wet tank fill up with water, which is why it rusted out. I drain the moisture frequently and hardly ever get any liquid water out of it. The 120 volt electric compressor fills the wet tank, which is supposed to have the moisture drained out of it regularly by the owner, so it really doesn't need a moisture separator. The problem is a lot of owners aren't aware they're supposed to drain the moisture out of the wet tank. I was one of those people until Stewart & Stevenson discovered the wet tank was full of water and showed me how to drain it. They drained a huge puddle out of it for over a minute!

Based on what I've read on this forum newer coaches with slides have small 12 volt DC air compressors that fill the dry tank(s) to hold the slides out while the coach is parked for extended periods of time. Because they fill the dry tank instead of the wet tank they need an air drier. A lot of people on this forum have been complaining about the short service life of these air driers and the frequent maintenance required, so I don't think I'll install one. I also wasn't aware of the huge difference in performance between the 120 volt AC compressor and the 12 volt DC compressor. I was previously planning to replace my AC compressor with a DC model but after yesterday's research I decided the performance of the large twin cylinder piston compressor with a 3/4 horsepower AC motor is worth keeping. I suspect it's powerful enough to run small air tools. I'll research this further. If it can run a 1/2" impact wrench that would be really cool. I'm pretty sure it can run a 3/8" air ratchet. I doubt the little 12 volt DC compressor would even be able to completely inflate a tire on my coach... Anyway, that's my thinking so I ordered the rebuild kit for my 120 volt AC compressor now that I appreciate it a whole lot more! haha

Re: searching for an air leak(s)

Reply #22 – September 13, 2014, 02:43:23 pm

@Scott I'd hold on to that 120v compressor. My 92' has the 12v and it's pretty worthless.

I also agree that you can get away without the dryer. Just be sure to check the drain petcock on a regular basis. Like you said the wet tank survived 20+ years under the worse conditions possible. I suspect you can get a replacement wet tank for < $100. At least that's my hope...

see ya
ken

Re: searching for an air leak(s)

Reply #23 – September 13, 2014, 05:46:20 pm

Yeah, hopefully the tank won't cost much. I may order it from the internet to save some money. I kinda think I need to take the coach to a truck shop with wheel lifts so they can put the coach up in the air and install the tank. I don't see how I can get the old tank out and the new one in by myself with the coach sitting on the ground. The tank is located in front of the rear axle on the passenger side. It's tough to squeeze my non-overweight body in there to even look at it. A truck shop will probably charge $100+ per hour I suppose. I'll have them soap suds check the whole system while it's up in the air to be sure I found all the leaks.

Re: searching for an air leak(s)

Reply #24 – October 02, 2014, 10:07:03 pm

Just to update anyone who may still be tuned in to this boring story, I overhauled the 120 volt AC compressor last weekend with a kit from the manufacturer, Gast, and it's working fine. I'm camped out at Freightliner of Maine in Bangor currently, where they plugged me in to a 20 amp electric connection, waiting to hopefully get the wet tank patched or otherwise repaired in the morning. Foretravel couldn't get the wet tank and said it'll probably be months before it's available. Freightliner seemed confident they could weld a patch on the bottom of the tank, or at least screw sheet metal screws with epoxy on them into the leaks to hold it until Foretravel can get the correct tank. It's also possible Freightliner can install a small tank, like today's trucks have, if they can't repair the existing tank.

The Gast compressor and rebuild kit is very interesting. I've never rebuilt an air compressor before but I have rebuilt engines. This compressor is a "Boxer" twin cylinder I think. It has horizontally opposed cylinders on separate connecting rods offset slightly. The pistons act opposite each other; by which I mean they travel out to full stroke together, then travel back in together. The pistons are inherently balanced but the motor does all of its pumping at the same single point in each revolution, just like a single cylinder compressor. The pistons have dual ring slots with thick outer rings, possibly nylon, over thin sheet metal inner rings, possibly stainless steel. There are a total of four rings on each piston if this is making sense to you. A nylon ring over a sheet metal ring in each ring groove of the piston. The compressor is sleeved, but the sleeve is around the piston instead of inside the cylinder. The sleeve may be graphite impregnated nylon and it has a diagonal slit around it so it can be installed over the piston. The entire cylinder head and piston face was covered with what may be graphite that came off the sleeves. The iron cylinder walls were perfectly mirror plated even though the pistons were loose and sloppy from 20+ years of wear before I overhauled the compressor. After the overhaul the pistons were tight and the compressor runs quieter. The cylinder heads are aluminum and they have dual reed valves, possibly stainless steel. The kit came with new reed valves and cylinder head gaskets in addition to the aforementioned rings and sleeves. There were also a couple of mystery parts that I didn't see in the compressor and the instructions didn't say what they were, so I didn't know what to do with them. Maybe they apply to a different model or something? The Gast compressor is obviously a heavy duty model that's completely rebuildable. After I got the compressor out of the coach I could see how big and heavy the thing really is and how ruggedly constructed it is. It's got a 3/4 horsepower 120 volt AC motor on the twin cylinder compressor... and it sells for $700 on the internet.

...I also replaced the spring check valve with a new one I got from NAPA. The old check valve was pretty messed up and didn't work at all, keeping air pressure on the compressor even when it was off. I know this because one of the cylinder head gaskets was leaking air while the compressor was off. So I'm glad I spent over $100 to rebuild the 120 volt AC compressor instead of installing a wimpy 12 volt DC compressor. The Gast compressor is apparently rated for at least 100 psi, but mine is set to about 70 psi for some reason. I'm probably gonna leave it set that way. It's possible I might add a 12 volt DC compressor at some later time, but maybe not. I'm starting to appreciate why Foretravel chose this Gast compressor now that I overhauled it and did some thinking about it. Now tomorrow I'll start thinking about why they installed a large wet tank that takes several months to order... There must be a reason that I haven't thought of yet, right?

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