Auxiliary Air Tank Leak

[Depewtee]

Hi All -

About six weeks ago I was taking the MC-9 out for my monthly trip around the block and noticed as I was driving I could hear an air leak (it sounded like it was right under me).  I was literally a block from the house and already pointed in the right direction so I kept on driving - the air pressure was still high.  As I approached my driveway I heard a big "pssssss" and the air pressure started dropping.  By this time I was in front of my house, but I still needed to back the bus into my driveway.  I did a quick search and did not find the leak.  I was blocking the road, so I placed the bus in reverse and overpowered the park brake.  The next day, still with no air pressure, I tried to move the bus and I could go backward, but not forward.  Unfortunately backward did not help much because the Prevost was already backed in as far as it could go and I was only about five feet from it.  Unable to drive up on my ramps, I was destined to jack the bus up and put it on blocks.  I managed to get it up and secured with my ramps and some cribbing.  I hooked my garage air compressor to the bus to help troubleshoot the source of the leak.  I determined it was coming from somewhere around the two front auxiliary air tanks.  I stuck my hand above the streetside tank and felt air whooshing out.  At this point I was pretty bummed.  There was a manifold on top of the tank with four lines going into it and an additional line on the side of the tank.  I disconnected the side airline, loosened the mounting straps for the tank, and tried to figure out how to get to the top lines.  There was no way to get to them and no access door to reach in from above.  So, the bus sat for six weeks until I finally had time  today to tear into it.  I managed to wedge my body up into the suspension area of the bus (everything was blocked real nice at this point, but it was still a scary feeling) and reach the first of the four upper airlines. 



After I disconnected it I tried to place my wrench on the second fitting.  That was not going to happen.  As I was contemplating my next move, I tried to move the tank a little to get a better angle on the second airline when all of a sudden the whole tank spun.  It was a eureka moment!  I was able to unscrew the tank from the manifold, although it was a pain in the cramped quarters and with the fittings (four of them) on the side of the tank catching on everything as the tank spun.

I dropped the tank and slid it from under the bus. 



I inspected the area where I felt the air whooshing out and noticed the seam between the top of the tank and the sides of the tank was starting to split.



I used a wire wheel brush to clean the area and then took the tank down to the local muffler shop.  I asked the guy if they did general welding and he asked me what I had. I told him an air tank with a split seam and he said bring it in.  I was in and out of there in less than five minutes.  He did not charge me, but I insisted he take a ten for his troubles (it was the only cash I had).  Hopefully, he is enjoying a good dinner or cold beverage right about now.

After


I brought the tank home and hooked my compressor to it and it held air.  I took the wire wheel to it, shot some paint on it, and replaced the rubber that went between the tank and the tank brackets.  Now comes the really fun part... shoving this back up where it came from.  Did I mention it was close quarters, I hated being so close to the suspension parts, and that this tank is pretty heavy when you are trying to suspend it in air with one hand?

Somehow the tank has to go back in here.


It took many attempts, but I finally managed to put the tank back in place.  I once again hooked my shop air to the bus and was able to get it to about 90 psi.  I still had a leak, but at least it was not coming from the tank.  There is valve assembly very near the tank that is supplied with air from the tank and then sends it back out somewhere.  It was dark by now and I am saving troubleshooting that leak for later, although I did manage to reach up to the top of this valve assembly and hand turn a "knob".  Turning this knob nearly made the leak go away to the point that I could not notice the needle on the air gauge dropping.  Tomorrow I will crank the bus and see what happens.  I really want to get it on top of my ramps (the wheels on top, not the way I have it now).  I will keep everyone posted.  If anybody knows what that valve assembly does I would love to know.

Brian S.

[prevosman]

I have zero knowledge of an MCI, but if in fact the tank is an aux air tank you likely have other issues. On coaches I do know there is likely to be primary air (rear brakes), secondary air (front brakes), and aux air for anything unrelated to the brakes such as suspension, horns, etc.

The coach should have protection valves so a total loss of aux air might drop the brake systems down to 75 PSI for example, but no less and you should not have had to overdrive the brakes. If an MCI is like that check the pneumatic systems protection valves and check valves.

[Depewtee]

Prevosman,

My terminology was incorrect.  I just took a closer look at my maintenance manual and the tank I was working on is called the "park tank."  The valve assembly mentioned is an "'e' type filter."  At least I think that is what I was working on....

Brian S.

[zubzub]

If the valve that "protects" the air system from auxiliary loads fails, you will loose your main air to the aux tank.  That said was it the aux tank you "fixed"* or just a front tank?
on  older GMs fer instance the tank  under the bus near the front is a front air tank, and the aux is accessible through a side hatch, older Prevs as well.  Sounds like your aux presure protection valve is a little off, they are replaceable fixable.  
Don't think you should be able to move your bus with no air  if the brakes are adjusted properly.

* I have always heard you should not weld a pressure tank....ever.  Unless you are a certified p tank welder and the tank is x rayed yada yada yada.
The danger is a high energy explosion with tank shrapnel....tanks are replaceable.
 I had to replace 3 on my bus, front aux and front main rear main, all had ruste through at the tank drain because of improper maintenace.  Found a front aux in another  bus, and used a couple of newish truck tanks for the other 2.  With a little creative plumbing many tanks will fit in a bus.

JUst read  your other post... guess you have DD3s ...I take back the properly adjusted brakes comment.  DD3s  will fail if no air in the tank reserved for them ( which is why I think they are stupid).

[Depewtee]

I guess I will be on the lookout for a replacement tank.

Brian S.

[prevosman]

Ditto on what zubzub said about welding. In fact, the most likely tank failure point from my perspective is where rust occurs under the straps. You may wish to look at all the tanks that are supported in that fashion.

All welding is going to do, assuming it holds, is create the next leak at the next weakest point. The extent of your repairs is likely going to be controlled by how long you intend to keep the coach because I suspect replacement tanks will not be cheap.

[Marcus]

I had my auxilary tank leak at the straps that secure it. 1982 mc9 I called Luke at US coach and he sent me a good used one for somewhere between 2 and 3 hundred dollars. Marc

[Joe Camper]

I have come across this failure more than once.

On Prevo about 98 or 99 they started adding small football size tanks in conjunction with each airbag. They sprayed them with the "spray foam" for the under-carrage (overspray without putting any primer or paint on the tanks themselves first) THEN painted the undercarriage along with everything else down there black on top of the foam. The spray foam is like a sponge to water.

Very lame for the worlds greatest bus chassis IMO.

The first thing they install on a Prevo after the wheels and drivers seat is the air leaks.

[Oonrahnjay]

I guess I will be on the lookout for a replacement tank.  Brian S.

  I went to truck parts supply place in Charlotte NC ("We have everything for trucks") and asked if they had a tank for my wet tank.  They didn't but they recommended a little place down the street that had a "universal brake tank" for about $69.  It was about 19" long and had three useable fittings, all 1/2" pipe thread, (plus drain on the bottom).  The tank that I had that had rusted out had about half a dozen 3/8" and 5/16" fittings.  I guess I could have made a couple of "Christmas tree" fittings and used the "universal" tank but that didn't thrill me (plus the stock tank had a large inlet at one end and the outlets were out the other so it would have meant a lot of rearranging hoses to match the universal tank).  I took my stock tank to the International Truck dealer, they said "I dunno, lets walk out in the warehouse and see if we have anything that matches" so we did but nothing was right.  Then I took it over to the CAT place - the parts guy disappeared with the tank and came back and said "nope, can't match that".  So I went to the Volvo-Mack place, put my old tank on the parts counter and the guy said "I think so".  He came back with a tank that was about right.  I had to re-route my towing/shop air fitting from the front to the back of the tank and replace the small line to the pressure gauge so it would reach a fitting on the rear but with those little mods, that tank worked fine.  It was about $224.  It was about 6" longer than the stock one so I gained some capacity (the stock brackets had rusted badly so I had to build new ones so that didn't matter).  PS. The new tank also had the drain fitting at the "corner" of the tank instead of pointing straight down so that gets the drain valve a little bit out of the way of getting hit underneath.

[Bussman84]

 Brian, I had to replace one of mine up front about a month ago picked up a used one from Caylor Supply in Rantoul Ks. for  around $100.00 Super nice people and they ship out orders everyday. Might be of interest, just do a Google search and it will get you a phone number and other info.

                      Billy

[Depewtee]

Billy,

Thanks for the tip.  I will give them a call.

Brian S.

[robertglines1]

Could have just been a bad weld from factory that failed.  Where it was located that would seem more likely.  Just my experience.  You will have to judge that.   Bob

[Depewtee]

Bob,

The tank was in great shape - no rust.  The air was leaking from the seam formed by the top of the tank and the side of the tank.  You can see how the weld is much thicker on the other side of the tank.  By the way, this was the top of the tank, so while I guess condensation could form up there I really do not see any signs of rust.

Brian

[luvrbus]

You can buy those air tanks from Eastern Marine from 40 to 80 bucks that was the price the last I bought anyway

good luck

[bevans6]

reading everything again I finally figured out what tank you are working on! It is indeed the emergency/parking brake tank.  It is fed from the dry tank, through an air filter and the pressure protection valve, and it will apply the emergency brake if it falls below the spring pressure in the push/pull valve.  If the pressure protection valve was working you would still have retained around 60 psi in the dry tank, which is the main supply for all the service brakes.

MCI tanks are a fairly unique arrangement but I don't see why you couldn't get a standard replacement tank and fit it up.  It stands vertically and the drain is about an inch up the front side, not on the bottom.  If you had a visibly bad weld, then a repair might be OK, but I would probably try to fit together a new tank.  The accessory tank is  under the drivers seat.  The tank beside the  emergency/parking tank is the wet tank.

Brian