You can buy the whole maxibrake assembly including service brake portion ready to go then you don't have to change the service pancake. you will have to cut the push rod to proper length to match up with clerances for the slack adjusters, don't know the price differential. If you decide not to upgrade to spring brakes at this time would still recommend replacing the pancakes if it hasn't been done in recent memory. They can and will fail. If one fails you would still have some brakes but you lose airpressure fast and could cause brakes to lock at an inopportune time amd your aircompressor would have to work a lot harder. As koolkar said don't under any circumstances take apart the top chamber of a maxibrake system.
