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1991 1250L Doka Unimog, 2002 ML320
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Discussion Starter · #1 ·
Howdy All,
I've noticed that it now takes more gauge pressure (7+ bar as opposed to 6bar) for the parking brake to fully release. It used to move immediately at 6 bar and now at the higher pressure one side drags a little. I don't hear air leaking. Any ideas? Thanks,
Chas
 

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U1600AG + a LR 90
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995 Posts
It can depend on the length of time parked up & the pressure maintained in circuit I & II & the auxiliary consumers circuit which will denote the state of pressure regulating valve’s thus the peek gauge pressure sequence that the handbrake will release (also the auxiliary consumers like 4wd, air seat, 4th/5th gear shift, etc)
 

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1991 1250L Doka Unimog, 2002 ML320
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1,406 Posts
Discussion Starter · #3 · (Edited)
It can be parked for a few hours or day. It has been a gradual thing I think. I'm curious where to start. Could it be the "unloader" or a bad bag on the rear brakes or something blocking the lines? Could it be a leak, even though I don't hear a hiss? I'm trying to get an idea if you all have had a similar issue and I'll tear into it from there. I've had the truck about 5 years now and it would fully release the parking brake at 6 bar or less. I used to have to be careful because the 4-5 air shifter has always needed about 7 bar to work and it wouldn't shift. Now my "hold-up" is the parking or rear brakes.
Chas
 

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Unimog U1250
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383 Posts
It can be parked for a few hours or day. It has been a gradual thing I think. I'm curious where to start. Could it be the "unloader" or a bad bag on the rear brakes or something blocking the lines? Could it be a leak, even though I don't hear a hiss? I'm trying to get an idea if you all have had a similar issue and I'll tear into it from there. I've had the truck about 5 years now and it would fully release the parking brake at 6 bar or less. I used to have to be careful because the 4-5 air shifter has always needed about 7 bar to work and it wouldn't shift. Now my "hold-up" is the parking or rear brakes.
Chas
Have you found any clues regarding this?

I'm interested as my truck is doing the same. I can only release the hand brake at 7+ bar pressure. I found a couple of very small leaks from the rear but fixing those didn't fix the problem.
 

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1991 1250L Doka Unimog, 2002 ML320
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1,406 Posts
Discussion Starter · #5 ·
I'm going to remove and clean the Gizmo that is squarish and has 4 large air lines going to it (I'm very technical). Ben and Bob have been giving me ideas and I'll go that way. I was to do it Saturday but I went and pulled a wrecker out of the mud instead. I hope to do it this week. I'll post what I find.
Chas
 

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1991 1250L Doka Unimog, 2002 ML320
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1,406 Posts
Discussion Starter · #7 ·
Howdy All,
I took the "Four-Circuit Protection Valve" apart today to see if it was gummy. Earlier I called it the Square Gizmo thingy. It had some nasties and the pressure adjustment screws needed heat and lube to become adjustable again. I totally messed-up all the adjustments, lubed it and put it back together. Still have the same problem. I'm attaching some pictures so folks will know what the part looks like. In the pictures I've already loosened the bolts and the top is raising up against the springs. On the good side......I now know what this part looks like inside but it would be nice if I knew what port fed what.

I wonder what a bad Parking Brake acts like? I noticed a valve after the PB and wonder if it is in there. Once again, any ideas.
Thanks,
Chas
 

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1983 U1200 (parted out), 1979 U1700L
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892 Posts
Bring back an old topic,

Dose any one have an exploded view of the 4-way protection valve, mine is leaking (I can hear it but have not gotten under the truck to check it out closer) I have a full truck manual but it dose not have an exploded view of the valve. Judging by the photos above, I will pull the front cover off of it and see if there is crud inside it, but a diagram would help too.

thanks
 

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Sold .....1987 U1250 Unimog
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1,011 Posts
Its pretty self explanatary once you pop the top off... 4 springs, 4 white cups ETC... nothign complicated from memory... just pay attention and reinstall the same way :)
 

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U1600Ag
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1,357 Posts
Bring back an old topic,

Dose any one have an exploded view of the 4-way protection valve, mine is leaking (I can hear it but have not gotten under the truck to check it out closer) I have a full truck manual but it dose not have an exploded view of the valve. Judging by the photos above, I will pull the front cover off of it and see if there is crud inside it, but a diagram would help too.

thanks
Could it be a leak coming from one of the airlines connection to the valve instead ?

I got a slow leak coming from those fittings. When wiggled, one of these hisses louder.


The third pic shows another leak that I found while washing the truck.
 

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1983 U1200 (parted out), 1979 U1700L
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892 Posts
I sprayed the whole area with soapy water and did not find any bubbles, I am going to pull the tire off the truck tomorrow to give me better access to the area. Refill the air system and get my head in there and see if I can narrow it down.

If its the 4-way I will pull it apart and see if I can repair it.

Is there a gasket between the top plate and the body, that I should be wary of?

thanks
 

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1983 U1200 (parted out), 1979 U1700L
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892 Posts
Fixed it, the valve was leaking from the back of it. I pulled off the front of the valve and cleaned and re greased the inside. But there was no way to get the rear chamber from the front cover, so I had to remove the 8 lines form it and pull the valve off the frame. Once off I found there is a 2" plug on the back of the valve, held on with a snap ring. I removed the ring and found it was very rusty on the inside and after wire wheeling it, there where pin holes in it. So I got a piece of 1/4" AL, cut out a circle of it, cut a grove for the O ring that is behind it and put it all back together. Leak is now gone.
 

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'91 U1250 '02 U500NA
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3,781 Posts
Hello everybody.
Now I'm getting this kind of the problem.
It takes more and more time and pressure to fill the tanks that are responsible
for retracting the parking brakes.
The control light doesn't even come off.
So what was the solution - servicing that 4 way gizmo ?
Chas did you find the problem ?
 

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1983 U1200 (parted out), 1979 U1700L
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892 Posts
Valve was easy to service, take it off, open it up and clean it all out. I used Vaseline to rebuild it.
 

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1991 1250L Doka Unimog, 2002 ML320
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1,406 Posts
Discussion Starter · #16 ·
Howdy Yarr,
I had a couple of issues. One was the 4-way control valve. The other was the rear parking brake portion of the calipers. Mine would take more pressure but the light would go off like it should. You might be having a different issue if the light isn't going off yet the pressure is beyond the typical point.
Let us know,
Chas
 

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'91 U1250 '02 U500NA
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3,781 Posts
The control light goes off but after a while.
I can hear the air filling the cylinders when I release parking brake,
but it's not as rapid as used to be.
I didn't touch $ way valve yet.
Hopefully tomorrow.
 

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'76 1300L,HE351CW,H15P Winches,Konis,Hydraulics,All Gears,10mm Plungers,Aftercooler,Lots of Littles
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3,151 Posts

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'76 1300L,HE351CW,H15P Winches,Konis,Hydraulics,All Gears,10mm Plungers,Aftercooler,Lots of Littles
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3,151 Posts
I know, but they cost a bloody fortune, so the "$ valve" seemed like a good name for it!

C.
 
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