[cir287]

Hi all, did a search but can’t find this issue -
started up the unimog yesterday for the first time in ~3 weeks. Air pressure built at usual speed to 6.5bar but just stops at that.
No air leaks I can hear other than the foot brake valve which has always had a slight leak. Belt is tight.
I took the head off the compressor, nothing looked amiss other than a little gunged up. Bore looks ok.
Cleaned all and reassembled but no difference at all.
Any ideas, is rebuild kit my best/only option or is there something somewhere I’m missing.
Thanks
Cir

 

[JeepinMike]

My money's on something downstream leaking. 6.5bar sounds close to where the 4 way protection valve starts feeding air downstream to the different circuits, and that downstream leak's preventing it from building past 6.5bar. Its can be a difficult one to find due to the leak stopping as soon as the engine is shut down....

 

[cir287]

Thanks for that Mike, I’ll try look a bit deeper so.

 

[DokaTD]

A large leak won’t hiss like a smaller leak. In fact a really big leak may not be easy to hear at low pressure

 

[NhBen]

If you can fill the tanks from a shop compressor it's easier to find leaks without the sound of the engine running.

 

[photobldr]

Okay, question: When you say “just stops at that”, what actually happens? Does the unloader/ regulator (first device that the air from the compressor touches) open and you hear a “whoosh” as it dumps the compressor output? that would indicate the unloader/ regulator is messed up and opening early. If there is no “whoosh” at 6.5 bar, then that would indicate either the compressor cannot generate any greater pressure, or there is a significant leak downstream from the unloader/ regulator. As per Mike, the downstream leak is more likely, and it will be a big one to pass all that air.

Is this truck a 7-8 bar system, or is it a 7 & 10 bar (for the two brake circuits) with an 18 bar cutoff?

Two things to try:

Run the truck, get pressure up to the 6,5 bar, shut it off and watch the gauges for how quickly the pressure falls off. If drops fast, you need to get the soapy water and start exploring.

Get an air compressor that will go to at least 8 bar, 10 is better, and fill the truck system through one of the air check ports. There should be a capped port on the 4-way, that you can get adapters for, and fill just after the unloader. That is as close as you can do to testing the entirety of the downstream connections.

With remote filling, truck not running, it is far easier to hear leaks, and generally search for issues. These leaks can be very small, very hard to find, and can be anywhere. Dish detergent and water mix in a spray bottle, and flashlight. Might get lucky and spot it right off, but don’t count on it.

Lee

 

[cir287]

Right so, heeded the advice and had another look today and as per above advice a big leak makes small noise.
Turns out the lhs park brake actuator is leaking.
I had started the engine with the brake off yesterday, the actuator was just bleeding air off when it reached 6.5bar.
I put the brake on and it built air pressure to ~15bar, cycled it a few times and all works albeit on borrowed time but at least I can move it now.
I have t looked at the circuits too closely but it is a dual circuit system. Both circuits were only getting ~6.5bar. Can that be caused by the brake actuator alone ?
Thanks all.

 

[photobldr]

Yes, if the leak is big enough to drain out the total input volume at that presssure value. A bit slower leak, the tanks should fill above that pressure value. After that, the park brake circuit should not be able to drain the other circuit/ tank, unless the check valve fails.

Lee

 

[DokaTD]

I have a decent leak on one of my brake cylinders. They are hard to hear. Makes my truck hit or miss on air pressure. It always disengages but the air takes time. I bought a fresh set from Teksan for a shockingly reasonable price. I just have to get out some knackered bolts.

 

[krietpiel]

My bolts were badly rusted - replaced them with new Allan cap parts - easier to work with given the constrained space.

 

[DokaTD]

Mine are Allan from factory. Or at least I think they are factory. But one or two of mine failed while trying to break them free. Will be fun trying to get them the rest of the way out.