I compared the tailor made snip of black rubber or neoprene that came with a MC at the parts house & it was identical to a piece of common black rubber vacuum hose off of their hose reel except for a shallow groove along the length of it I'm assuming for air to excape so it can go all the way down into the bottom of the piston recess & lock in solid. I've went that route several times & it has always stayed locked in their super tight. It does require that the rear piston tapered "thimble" recess has the circular shallow groove about halfway down the barrel for this piece to lock into. Power brakes can use a MC with a recess or no recess (of the right bore of course) but manual brakes require it if useing the snip of rubber or use some other method of retaining the pushrod as mentioned. Ehrenberg even used some RTV to lock the pushrod down in there on a piston without the circular groove. I have yet to try that!