I used to work on the Bendix assembly line building these things way back when. I was the line repairman that got to troubleshoot and repair any units that were rejected on the automatic test stands.
That said, yours may be very similar to what I often found back in the day. During assembly (the line workers were paid by how many were built, so they emphasized "speed" versus "accuracy", hence why I was so busy!) the line workers would often accidentally insert TWO reaction discs where there was only supposed to be ONE. This made it impossible to adjust the push rod properly. It was always to long. Sounds like what you have and the fact that you said you sent it in to be rebuilt recently also tells me this could be your problem.
The solution? You have to take the push rod out of the booster to check for an extra reaction disc inside. To do this you have to remove the rubber seal around the push rod from the booster. It probably has a metal external tooth like spring washer that is pushed down over the rubber seal to keep the seal in place. So, basically you have to just grab the push rod and pull up/out while maybe also encouraging the spring washer to release with a pry tool of some sort. (Screwdriver?) Back then, I had special tools on the line that I could just pop that push rod assembly right out without much of a hassle, but you can get it out with common tools just be careful not to damage the rubber seal since you have to reuse it of course. (of course, I had lots of extra parts too when I worked on the line so I didn't have to worry about the rubber seal so much.)
After you have the push rod out, the reaction disc may or may not come out stuck to the bottom of the push rod. If it sticks to the bottom, use a flashlight and check inside carefully for a second reaction disc still in the unit. Otherwise, look for TWO discs inside. They will fit inside a round hole in the center of the diaphragm plate inside, but it is possible that it will pop out when you take the push rod out, so be sure a second disc isn't floating around inside the booster that you don't see before you reassemble it. BTW, these reaction discs are solid rubber approximately 3/4" in diameter and maybe 3/8" thick. So, hopefully this will help you but I know its only a "shot in the dark". Good luck!
