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Thread: FLU419 air compressor zero PSI; where do I start?

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    Default FLU419 air compressor zero PSI; where do I start?

    new to me FLU419; pretty rough shape.
    in trying to diagnose my zero brake issue through searches here, I soon realized my air system is at zero psi.
    Sounds like the engine (cam?) driven compressor runs all the time.
    Then a system of relief valves and regulators keep the system around 100 psi.
    Systems include: brakes, 4x4, transmission, lockers, trailer brakes, tire inflation?...

    So... where do I start?
    Do the compressors ever go... bad?
    Should I delete some of the extra systems that will never be used? ie: trailer brakes, alcohol injection (it does it get to about 0*F, but rarely much lower)

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    First question i have is... zero brakes? Yes the brakes have an air assist, but itís only an assist. You should have somewhat effective brakes with zero air pressure. If your brakes donít work at all, the hydraulic brake system is the first thing that needs fixing.

    Yes, thatís basically how the air system works. Though I think itís driven off the crank.
    Last edited by Pinsandpitons; 05-12-2019 at 14:06.

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    yah... the complete lack of brakes has me worried. Master was supposedly replaced, but with no hood it's been rained on quite a bit.
    Should I assume that aside from the booster, the brake system should be treated like any other system?
    1- Look for hydraulic leaks.
    2- Bleed system.
    3- If still no pressure, replace master.

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    I would suggest not going directly at the compressor. Go the misc. tech manuals forum and download the student guide. It has the best air and brake system drawings out there. Now find the air line from the compressor located between the front and rear wheel on the passenger side. Break the line on the inlet of the alcohol injector and see if there is pressure in the air line when the engine is running. If there is, move down to the outlet of the air pressure regulator, that is a low point in the system and is prone to building up crap. Some SEEs have a built in screen on the inlet some do not. The regulators are available on Ebay on occasion by searching for the part number. Somewhere on the long Unimog thread is discussion where others have bought them. Expedition Imports has them. Now check the cross body air line, its a known issue that the line rots from the inside out and creates rust flakes that carry forward in the system. The military fix is replace it with stainless steel tubing. I used cu/ni tubing as its lot easier to work with.

    Note in order to break the air fittings you need appropriate sized wrenches. The fittings are not rugged and you need to back up every fitting with another wrench to make sure you don't stress the wrong joint. The SEE uses multiple metric thread types so getting replacements is not fun.

    If the cross body air line is not plugged or rusted out, now start taking fittings apart in the air tank area and clean all the gunk in them and remove the air tanks and clean them out. Reassemble and then see if you can air up the tanks with the engine compressor.

    Note if you decide it is the compressor, rebuild kits are available or entire units from Expedition Imports. I think you may need to tilt the cab to get at it. You need special brackets to tilt the cab that one of the forum members makes for Expeditions Imports.

    Others have backfed the air system with separate air source to see if they can find any leaks.
    Last edited by peakbagger; 05-13-2019 at 05:35.
    Unimog SEE, Unimog 1300L Ambulance

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    Great info; thanks.
    So... progress.
    The compressor... compresses.
    I found both holding tanks leaking out of their drains. Pulled, cleaned, returned... and now it's a little better.
    Oddly... only the smaller tank is getting air. The air line from the main pressure regulator comes across to a "T" on the drivers side. It feeds into the small tank and into the larger tank via a second regulator (or pressure relief?). I suspect there must be a screen full of debris preventing the main tank from filling. And it seems the main tank is the more important one.
    Some previous owner (Uncle Sam?) covered everything in beige house-paint; making everything just that much more fun to work on.

    Anyway... progress. Slowly. Thanks again for the help and encouragement.

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    Congrats on your progress. If there are no leaks, the compressor should pump up to a certain setpoint and vent air from the regulator every so often when the pressure goes over the high setpoint. This in theory drains any liquid out of the lines upstream of the regulator. If its not "spitting" then you need to figure out why not. FYI check the 4wd lever to make sure you are in 2WD. In 4wd the axles are pressurized and there can be air leakage if all the tubing is not tight.

    I agree that military green CARC coating is tenacious stuff, not sure if the tan is as bad but expect it is. It either epoxy or polyurethane and not many solvents will strip it off as its designed to be steam cleaned to pressure washed. The green has to be chipped off. I managed to break that Tee and its a weird one with two different thread types, I could not find an exact replacement. I had to do some substitutions to replace it. I left mine stock but I think one tank is for the SEE brakes and the other is for the external trailer brakes. I think others have bypassed the trailer brake tank. My theory is might as well just clean it all. If you printed out the air system diagram I expect you could figure out which is which. Unless the cross body air line is stainless (check it with a magnet) I would suggest disconnecting at either end and blowing compressed air through it as its low point and I expect there could be reservoir of crap in it. Best get it clean first and work out from there.

    I got lucky and the crap didn't get up to the brakes and various regulators and actuators so someone else will have to lead you from here.
    Unimog SEE, Unimog 1300L Ambulance

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    Update: changed the fuel filters because engine barely had enough power to get up my street. Stalled on a tiny hill.
    Filters full of mud. I guess not having a fuel cap will do that. 🙄

    Iíve been hesitant to attempt my very steep long driveway because if it died... without brakes, It would be stuck blocking my house.

    But I went for it. And as I slowly drove the 1/4 mile or so to the top of the hill... the air pressure gauge started to show pressure! Sweet.

    THEN... just as I got to my clearing... the engine decided to accelerate on its own!! My best guess is the remote high idle was on, the air pressure finally activated the air solenoid. Well my first thought was brakes.
    No brakes.
    Second thought was ďyou should sh** your pantsĒ.
    Third thought was key off.
    Still accelerating at a line of trees.
    Put clutch in (duh!). Engine finally died.
    Then I started to roll backwards back down the hill.
    Luckily was able to get it into a gear.

    After I change my shorts... going to go find the idle down button.
    Any clues where that is?

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    Please read the operator manual.
    Then find the pneumatic throttle control within arms reach of the right hand side of the backhoe operator.
    But seriously, please read the operator manual.

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    I agree,read the manual and do all the preventative maintenance on the SEE before driving it. You could be causing damage that you will regret later. The lube routine where you start at one point and then rotate around the vehicle is definitely worth doing. As an example, the portal axles only contain a very small amount of fluid, if that fluid is low or contaminated and you screw up the portal gears and bearings, you are looking at hundreds is not thousands of dollars of repairs.

    The emergency brake works quite well as it uses the same rear calipers and completely independent of air and hydraulics. Definitely worth making it a priority.

    In addition to the pneumatic throttle control on the backhoe, I seem to remember that when the hydraulic tools circuit is turned on the pneumatic throttle is engaged. The other thing to check is the throttle linkage, its very complex and easy to be damaged or out of adjustment. It seems to be a common problem with SEEs that come out of surplus. Since it sounds like you may need to tilt the cab to trace the brake issues (after you have ruled out the master cylinder, you may want to wait on going through the throttle linkage as its easier to do it with the cab tilted (with the exception of actual accelerator pedal).
    Unimog SEE, Unimog 1300L Ambulance

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    Yikes! Well Iím glad everything turned out OK with no damage or injury. I think it bears repeating that you should have reasonably effective brakes with no air pressure at all. So if you donít, then I would abandon for the moment any investigation or work on the pneumatic side, and focus on getting the truck to stop. You may have a simple leak at the caliper or somewhere else that just drained the system while it sat and bleeding it might return function and show you where that leak (if it exists) is. Safety systems first.
    Last edited by Pinsandpitons; 05-15-2019 at 09:25. Reason: Spelling

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