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On most all modern OTR trucks there are no serviceable parts in the spring brake can.  Do not attempt to open or replace diaphragms in any way.  You simply replace them, they are cheap.  

As an American I am hesitant to even post this but to have some information is better than trying it with no information.

Sometimes however finding new spring cans is hard.  Or maybe the truck is old, foreign or in this instance ex-military.  There are few if any easy options for these trucks.  Like working around split rim tires, work smart and methodically, treat what you are doing with respect and be aware of others in your work space.  

Officially my advice is *DON'T DO THIS* however if you plan to anyway, here are some tips, tips that should in no way be seen as full instructions just... tips.

Inspect the caging tool for damage and replace if you see ANYTHING that looks funky.
Do what I did and try to damage the caging bolt by whacking it by hand on something hard. If you mark it, replace it.
Chock it if you don't have mud to freeze it into. (Chock anyway)
Inspect the caging socket; get to know it with a light. If you see damage it is over, replace the whole thing.
Play with the tool to get the feel of the socket, make love to it with the bolt. You want to KNOW when you have it properly seated.
Lube the threads. JUST THE THREADS. I used wax. Lots of tension is going on there and you don't want the added stress of a galled up nut.
Set the parking brake and cage the can.
No air tools or socket wrenches when caging. Use an open end wrench so you can see, hear and feel what is going on.
Set and release the parking brake a few times and listen for any noises that might mean things are not right in the can. *ping*
Air down the truck.
Make sure the truck is aired down. That will mean opening valves 1,2 and 3. Leave them open, leave them all open, even 4.
Go check the air again.
Treat it like a loaded gun when caged, stay out of the line of fire, inspect from angles. No helpers.
Getting those diaphragms in was frustrating. It took a long time. Nothing is going to make that better. Expect it. Don't rush.
I think the tire rubber lubricant (the real stuff) helped center that diaphragm. I didn't show it because I don't know if it is good for it but I think it helped.
Clean the clamps before you install them. I lubed them, I think it helped.
Replace the clamp nuts with quality 3/8 stainless nylon lock nuts. You will thank me.
Tighten the clamps and tap them, tighten and tap, tighten and tap. Don't over tighten and damage the bolts.
Uncage, then immediately recage.
Re-inspect everything again and then loosen the clamps. I could hear and feel the rubber adjust itself.
Tap and tighten, inspect and uncage.


I used NAPA part numbers MBI DP12 and MBI DP16. I have yet to replace any of the service diaphragms but I wanted some on hand. Still looking for a decent price (or any) source for new (or rebuilt) whole cans. Recommendations (part numbers and store names) appreciated.

Don't hurt yourself.  There is a reason most people don't do this anymore.

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Bio: I am a teacher who enjoys environmentally responsible woodworking. Most evenings will find me in the shop working with my three year old son Shay ... More »
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