Unimig 4x4 series brake problem!!

[greaser]

My uncle has a 1966 Unimog 404 Series with air over hydraulic brakes that have recently been rebuilt. It is now leaking air into the hydraulic system, and after about 1/2 hour the brakes are locking up. He has adjusted the pressure up and down, adjusted the linkage on the trailer brake valve and is now out of ideas. WHAT IS WRONG? Any of you encountered this?

[coachgeo]

Originally posted by greaser
My uncle has a 1966 Unimog 404 Series with air over hydraulic brakes that have recently been rebuilt. It is now leaking air into the hydraulic system, and after about 1/2 hour the brakes are locking up. He has adjusted the pressure up and down, adjusted the linkage on the trailer brake valve and is now out of ideas. WHAT IS WRONG? Any of you encountered this?

Ill run this question by the Mog Technical mail list I belong to.

[greaser]

Thanks a lot! He's been having this problem for a while now and has come up short on all his leads. Thanks again

[coachgeo]

u say something in this about the trailer brake valve. Is he pulling trailer? Does a trailer have anything to do with all this? if so which one has brakes locking up the trailer or the Mog?

Im assuming ur not refering to a trailer being involved at all. below is the first responce I on the mail list. It does not sound like it will be too helpfull.

[-------From Technical Mog mail list-------]
"If the trailer has a "maxi" style brake on it it would stop when there is no
air supplied to it. I would try to cure the air leak on the trailer, as it
sounds like that's where the problem is. (The mog doesn't have this problem
when it is not pulling the trailer, right?)
I don't think it is leaking air into the hydraulic system, (Not positive, as
I have a 406)but there is no place in the system for the two to come in
contact, as they are separate systems."

[greaser]

He does not pull a trailer, so I'm assuming there's a valve on the Mog called a trailer brake valve. I will check to make sure. Thanks again for the help

[greaser]

I just talked to my uncle and this is what he said. The EMERGENCY brake is the problem, it is actuated by an "air over hydraulic" system. What is happening is air is leaking into the air side of the "air over hydraulic" system, and eventually building up enough pressure to cause the brakes to lock up, making the vehicle immobile. He theh has to exit the vehicle and bleed the pressure the move the vehicle again. Any ideas? Thanks

[coachgeo]

Hi-

My 1963 404.115 VLF (firetruck) also has air-over-hydraulic brakes.
The air assist is triggered by pressure from the hydraulic system. On my
truck, there are two valves where the air and the brake hydraulics
interact. They are under the truck, just forward of the rear axle. I
occasionally have trouble with the brakes binding, especially in hot
weather. It also happens more if I don't drive the truck often, and
clears up with more use. I've also heard from some other VLF owners that
they have had the same problem.

To make sure it is the air system causing the problem, I would
disconnect the belt on the compressor, and drive the truck without the
air assist. The brakes will still work, it just takes more effort to
stop.

Please let me know what you find out.

Rob

[greaser]

Thanks, that's the best lead I have gotten yet. I will have him try this and let you know the outcome. Thanks again

[coachgeo]

Yes, that's the same setup that my truck has. One valve has linkage
to the hand-brake. If I pull up on the hand brake, the truck sneezes, so
all four wheel's brakes are set.

The temporary fix is to remove the air pressure.

Long term fix would most likely be to rebuild the air valves.

Ron

I have found that sometimes with older vichels that the short rubber hoses at the
end of brake lines swell /expand thus keeping pressure on the brakes When I taught
Auto repair my students ( who all were driving cars/trucks on their last legs) we
got to see this syndrome several times.

Chuck

[greaser]

I talked to my uncle and he is going to try your idea. If it works, and he determines that the air system is causing the problem, what do we do next? You know people that have had this same problem? What did they do to remedy the problem? Thanks again for the help!!!