I haven't converted any thing that early to spring brakes but I have converted some older trailers to spring brakes and it was relatively easy to do. I would definitely take Koolcar's advice, especially regarding disassembling the brake cans. Going by what systems I've worked on, one thing you will have to change, other than the cans, is the valve which is usually mounted on the cross member by the axle. The single diaphragm valves that I've seen had a service line from the foot valve going in, an emergency line from the emerg./park brake valve going in and a supply line going in or sometimes its mounted right on an air tank outlet. Going out you have a service line to each diaphragm. On the dual diaphragm valves you have the same lines going in but you have an emergency line going out to each can as well as a service line. The service side of both systems operates pretty much the same, but on the single diaphragm system when you apply the emerg./park brake and evacuate the air on the emerg. line into the valve, it it applies line or tank pressure directly to the service line to the diaphragm. It works pretty good until your tank pressure leaks off, then your truck can roll away. On the dual diaphragm / spring / maxi-can system when you apply the emerg./park brake and evacuate the air on the emerg. line into the valve, The valve "dumps" the air on the emergency side of the can and releases the spring to apply the brakes. Your brake will remain applied regardless of the tank pressure in the system.
