My bad: The cap-nut is physically part of the threaded push rod that engages the mc. I had thought the cap-nut came off, but it does not. So my idea of using a lil nut to hold the cap-nut in place assuming the cap-nut was separate from the threaded part won't work. What about thread locker? Is that a bad idea to use in this instance?

Got your IM a little while ago Searcherr I've been ill since thurs and haven't been logging on here.
I'm not at all sure what you're asking me. The 7mm domed cap nut is an integral part of the brake booster to m/c clearance adjustment. I didn't see any need to add any other locknut or use threadlocker, nothing else just turn anticlockwise to extend and I've no idea how far but I brought it out up to 3/4" at one stage.