The pushrod from the rearbrake is a loose fit in the rear master cylinder and normally cannot come out when adjusted properly. But everything is possible in racing. So I simply glue (10 second glue) a piece of fuelhose over the hollow rod from the brake cylinder, and everything is "safe". The pushrod is slightly smaller in diameter of course but is kept in place bythe rubber hose.

Yamaha TZ350 rear master cylinder