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Posted by D Sherman [72.47.153.24] on Tuesday, February 28, 2012 at 00:14:40 :
In Reply to: OT: M35A2 Brake Booster Question posted by Joe Lorenzino [216.197.153.67] on Monday, February 27, 2012 at 18:57:19 :
Yes, the M35 booster, commonly called an "air pack" is air actuated. I couldn't tell you what size the cylinder is. With no air, the brakes will still stop the truck, but you'll have to work at it. I like that better than normal air brakes, where you're dead in the water if you lose air.
M35 air packs tend to be expensive and good ones are scarce, considering how common the trucks are. I think the main reason is that few people know you need to oil them. For some inexplicable reason, there's nothing in the lube order about lubricating the air pack. After 30 or 40 years, the o-rings have gotten dry and they don't seal well any more. Guys think they have to change out the air pack when all they really have to do most of the time is remove the 1/4" pipe plug on the back of the air cylinder and put a couple squirts of air tool oil into it. I've given several people this tip and in every case it had their air pack working like new again.
On the other hand, you might want to keep this secret and get a "bad" air pack for free from somebody and then miraculously "rebuild it" by giving it some oil. Bear in mind that all the newer M35s had DOT5 brake fluid in them, so you would need to make sure the fluid left in the air pack matched what was in your unimog.
I've put 10-15,000 miles on my M35s and have been happy with the brake system, even though none are really adjusted properly. I've had up to 33,000 lbs combined GVW, I've gone on a lot of steep mountain roads, and I once had to do a loaded emergency stop with a trailer that left me in a thick cloud of blue smoke from all the old oil burning off of the brake shoes and drums. Whatever the actual specs of the M35 brake system are, they are clearly adequate to the task.
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