This essay details the procedure I used to convert the truck's manual hand brake to an air operated emergency brake using a spring brake chamber (aka Maxi brake I think). The old hand brake on the truck needed two men and a dog to apply and three men and a dog to release it so I thought "Hmm all it's doing is pulling this here rod, if I use a spring chamber to pull the same rod life will be much easier". A spring chamber is basically a round device with a massive spring inside it and an actuator rod protruding from the middle. When the spring is relaxed the rod is in (or out depending on the model, more on that later) and when air pressure is applied the rod is out (or vice versa). Sounds simple doesn't it? The original design was something like the diagram below. Note that, as the chamber is applying force by pulling, the coupling between it's actuator rod and the existing linkage can be loose.

Fig 1.

So I bought a chamber, spent a few hours making brackets and linkages, got it all hooked up, started the motor, applied the brake and...it didn't work. The chamber activated all right, and it did pull the hand brake rod, but there was nowhere near enough force being applied to engage the brake. Looking closely at the setup I find that I can easily pull against the chamber's spring, that doesn't sound right, these things are supposed to be really strong. So I go back down to the truck spares place, explain my problem and say I need a bigger chamber. They agree and get one but that's too weak as well. Then someone says "You need the double chamber, it's got one spring for the service brake and another for the emergency brake". They get one and drop it on the counter. Now this looks much more like it, it is larger but more importantly it's covered in warnings that tell me there's some serious force contained within. I buy it and head down to the workshop. The new chamber has exactly the same mounting holes and rod length. How easy can it get?, I simply install it and Bob's your uncle. Fat chance, the emergency half of the chamber works in the opposite direction, it extends the rod when air pressure is removed, the other chamber retracted the rod. My carefully manufactured linkages where made for a device that pulls, this one pushes. What to do? I could move the chamber to a position behind the original linkage so it's action was in the right direction, but this would be a real pain. The other option was to reverse the action on the linkage, I chose this course of action. By extending the lever and attaching the hand brake linkage at the other side of the pivot point I reverse the action, ie. when the chamber pushes, the linkage pulls.

Fig 2. Modified layout with reversed action.

Here's a photo of the modified linkage. I used a piece of scrap steel and the extra holes where already in it. When I'm happy that the adjustment is correct I'll trim the link and finish it off.

Fig 3.

So far so good however the first version of the coupling was loose, which is to say that it was simply a couple of rods with plates that had holes in them. A pin through the holes connected the two rods. This is fine for pulling but no good for pushing, for the same reason that you can pull a chain but not push one. I did try using this coupling, it worked but the chamber's actuating rod has no lateral strength and the linkage buckled quite badly. I felt this was placing too much stress on the components. I needed a new, more robust, linkage so made one by welding a nut suitable for the chamber's actuator rod (these are threaded) to the end of a length of 20x20 square tube. At the other end I welded the clevis bracket from the original linkage. The result was a long tube with a thread at one end and a clevis bracket at the other. This tube is screwed onto the chamber's actuating rod until most of the rod is inside the tube. Now the chamber can apply a lot of force to the linkage without it buckling.

Fig 2.Detail of the new linkage. Not to scale.

Note that the new linkage still allows the actuator rod to "buckle" a tiny bit. However it is well contained within the tube. So that's that, it seems to work well. The new design has heaps of room for adjustment and is very easy to use. I have installed the control valve on the floor next to my seat and simply push it to go and pull it to stop. A few notes about this conversion. This is also an emergency brake, if you lose air pressure it will activate. Of course this means that you cannot set off until the air is up to a decent pressure which isn't a bad idea in itself. The control valve must be a special type that can vent the air from the chamber and that will drop out if the pressure is lost. This is not only a safety feature but I suspect a legal requirement as well. If the chamber doesn't have warnings like "Serious death or injury may occur when using this device", "Under no circumstances should the retaining ring be loosened without the spring being locked securely" or "To be serviced only by qualified personnel" then it probably is not strong enough. When you get the chamber there is a threaded rod protruding from the rear as well as the main actuating rod at the front. This rear rod and nut is holding the spring in compression and it must be removed before you can use the chamber. However you must apply air pressure to the chamber to do so, this compresses the spring as well and removes the tension from the locking device thus allowing you to loosen the nut the remove the rod.