Air pressure question - Benzworld.org

mogstick · 05-16-2008, 04:44 PM

I've fixed 2 easy to find leaks in the air system but there seems to be more that are not so easy. The main one was the trailer brakes. That's good now. The other was the seat. Now with both of those eliminated the pressure leaks down to 4 bars overnight but will stay there indefinetly. I figure there must be a pressure shut off or something that would isolate part of the system. I'm thinking that if I know what and where that is that it might help me narrow down the search. Does it make sense that my truck does this? Thanks!

Chas Stricker · 05-16-2008, 07:16 PM

Good question. Mine does the same thing. Any ideas?
Chas

60th LCC · 05-16-2008, 08:07 PM

It sounds like the air system has a minimum gate regulator that shuts off some of the system until 4 bars is reached and the leak is down stream of this regulated point. Designers use this trick to ensure air for must have saftey related needs in case there is a large air loss or demand in less important devices which will be located past this minimum regulated point. hope this helps.\Dave

**coachgeo** · 05-16-2008, 08:24 PM

a hose pinch off plyer might can be used to pinch off various rubber line ends and see if the leak slows even more. Pliers may not shut air off completely but if it slows a good bit after applying them then you know it's something downstream of that line.

Problem is there may not be much of any rubber lines to pinch off.

normalbloke · 05-16-2008, 11:50 PM

A garden sprayer or trigger bottle of water/washing up liquid and hose her down.It'll find the smallest of leaks!

vmax29 · 05-17-2008, 07:10 AM

I had problems with the brake regulator just behind the cab on my 77 doka. May be worth it for you to have a look with a flashlight. You can see the purge valves on the outside of the regulator look for tearing that allows water and debris in. Its a bit easier with a tipper bed tilted up.

Picasa Web Albums - vmax29 - Unimog Projects

If you page through the photos front and back I took pictures of the regulator torn down and the rebuild kit. Its not that hard to do but there are a few small parts and springs to watch for. The problem with mine turned out to not be the purge itself (that only keeps out the water) but a piece of rust or debris. It could have come from the purge or the air line itself. It was just enough to allow the knife edge on the inner seal that holds pressure to allow air to leak by. The pressure would leak down overnight. Since I fixed it I noticed the next regulator in series is leaking I may need to rebuild that one next. It now leaks down a bit then holds like yours. Up close after shutting the truck down you could barely hear the air escaping from this regulator.

Another photo showing the location of these regulators with the bed tipped up:

The one closest to you in this photo is the one I repaired. The larger one further away is slowly leaking now. It may just need a tear down and cleaning.

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Hope this helps.

Sean

hammogger · 05-17-2008, 03:22 PM

The device that holds the pressure is called an overflow valve. Typically this valve only passes pressure beyond it after brake (and other critical) system pressure is up to 5 bar or so. 4 bar sounds reasonable, too, because it's an adjustable device. If there's a pressure leak 'beyond' the overflow valve then pressure will leak down to the 5 or 4 bar and hold. BTW, I have three 406's and a 1300 that do this. I have considered this normal behavior and don't worry about it.

Bob

mogstick · 05-18-2008, 05:54 AM

Anyone have any idea where the overflow valve is on my U1000? I guess it's not really a big deal since the tanks dont completely drain anymore but it would be nice to find. I guess this means my leak is probably not in the brake system since the tanks would probably fully drain still? Thanks for the help guys.

vmax29 · 05-18-2008, 08:53 AM

Quote:

Originally Posted by mogstick

Anyone have any idea where the overflow valve is on my U1000?

I think the valve referred to as the "overflow valve" is the first regulator in the series MB part #001 431 29 06 on a Case tractor. Some mogs have the alcohol injector there (right behind the fuel tank). You will know where that is because once the system is up to pressure it pops off and dumps the additional air. I think the problem you are referring to is in your brake circuit regulator and that would not be a problem unless you are towing a trailer with air lines. Not sure of the manufacturer on yours but the regulator I rebuilt was a Bucholz and your favorite mog parts dealer can usually get a rebuild kit from Germany. These regulators are quite expensive so be sitting down when they quote a price for a new one. Makes the rebuild kit and your time worth the effort. If I am not mistaken the regulator losing pressure would probably look like the larger one with the white cap shown here:

vmax29 · 05-18-2008, 09:41 AM

Mogstick, I was in a rush here. You said in the original post that you fixed the trailer brake circuit. Other than that the only other places I could think to check are the bleed off valve below the auxiliary tank and the main service regulator but you would lose all the pressure if that were faulty. Air leaks are annoying let us know if you get it figured out.