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1980 Unimog U1000
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Discussion Starter · #1 ·
I gave a U1000 Ag, made in 1980. It's developed a handbrake problem which I would appreciate any help with please.

The air handbrake light doesn't go off and the brake remains on, after starting the engine and allowing the air pressure to rise to 8+ bar gauge. The footbrake circuit pressurises and the dash light goes out, as does the PTO circuit, with the usual air-pressure sound when they're operated, but there is no apparent air pressure feed to the handbrake circuit i.e. no sound of air pressure when the handbrake lever is operated. The handbrake just remains stuck on and my Mog is effectively rooted to the spot!

Thanks in advance for any help in pointing me in the right direction to fix the problem.
 

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U1450L DOKA
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14,744 Posts
I agree with the above. There is an emergency parking brake release procedure which is outlined in the manual.
It allows you to release the pressure on each rear cylinder in turn. This frees the truck up so you can drive it.

It won’t solve the problem, but it restores your mobility.
If you park it on level ground in gear, and possibly chock the wheels, you can use the UNIMOG as normal while you
Chase down the problem.

Your earlier truck might have the alcohol injection anti-freeze system for the air system, or it may have been retrofitted with the air dryer system which uses the canister. Either one of these can restrict the the pressure building up, but the 4-way valve is the first place to look.

Is your truck a 424 series ?
Do you have the manual ?

If you need it, I believe I can post the release procedure so you can get rolling at least.
 

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U1450L DOKA
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Here are the procedurefrom the 424-425 series,
Including the U1000.
There are two procedures, which are shown on pages
71-72.
The first procedure involves pumping air into the truck’s air system from an outside source. This information is contained in just one ‘panel’, the first panel on page 71.

The second procedure is illustrated over 5 panels,
taking up the remainder of page 71, and all of page 72.
This involves mechanically releasing the cylinder arms.

The full page photos show the panels in sequence.

News Font Material property Newspaper Publication
Publication Book Font Material property Pattern
Organism Font Adaptation Pattern Illustration
Publication Book Font Material property Terrestrial plant
Jaw Font Poster Pattern Illustration
Font Parallel Rectangle Paper Number
Font Pattern Terrestrial plant Circle Monochrome
Font Art Parallel Drawing Paper
Font Parallel Art Illustration Drawing
 

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1980 Unimog U1000
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Discussion Starter · #6 ·
Many thanks for your help NhBen and TRUKTOR. I very much appreciate it. I will check out that 4-way valve and the parking valve that you mentioned NhBen. I do have the workshop manual in a file on my computer but it's not in a format that can be electronically searched, so I would have had to wade my way through it to get to the gems of informtion you've both given me.
Speaking of which - thank you very much TRUKTOR for all the extra information and uploads showing the workshop manual instructions. To answer your question, my Mog is a 424 as far as I can make out. I now know what I will be doing this coming weekend - with a renewed sense of purpose!! Thanks again and all the best.
 

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U1450L DOKA
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The plaque on the end of your dash should have your
VIN, and 424 will appear there, as well as being stamped
Into the right side of the frame ( from the driver’s seat ),
on the outside of the frame rail, forward of the front
Axle.
It ‘almost has to be’ a 424, given the year and the model.

Also, the parking brake valve itself can get clogged up
With crud ( rust, etc. from the condensation inside the
Airlines.
Basically, any part of the air system can fail to function from the accumulation of ‘Crud’ inside the valves,
Etc.
One trick you can try ( it probably won’t work in this case)
is to push on the brake pedal a few times, even before the
Pressure is up to snuff. Then when it gets up to operating
Pressure, with the parking brake in the OFF position,
Stab the brake pedal 4 or 5 times. Sometimes this will
Induce the parking brake to let go. You can usually here
or feel it, and the red light goes out.
It worked for me at times on my previous truck.
 

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1988 U1300L RW1 Working gears Dual Tanks AC Rigged for Camping Plus: 91 F250 HD 4x4
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587 Posts
We have posted a bunch on releasing the parking brake for ulsulanautor in Chile. 1300’s have this. Contrary to manual, tire lever rarely works, and hammer swing is such that I highly recommend using a short piece of plastic pipe as a drift punch rather than whacking the ring directly. There is quite a jump when it releases, keep fingers clear of the rod and lever.

Lee
 

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U1000Ag
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There is also the charging valve. Mine is setup so it will not introduce air into the park brake system untill there is at least 4.5bar in the tanks. Here is an old pic of mine - well hidden behind the cab
Motor vehicle Automotive tire Wood Twig Automotive exterior
 

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