Home      Masking Tape Is Your Friend!
A simple trick to prevent telescopic brushes from accidentally retracting is to wrap some masking tape around the end of the plunger rods (if the design allows for this). The tape simply makes for a snugger fit when the end of the rod is rammed all the way home (stop tittering at the back).
 

Although other types of tape will last longer, masking tape is easier to remove and reapply (no need for scissors, just tear a piece off the roll). This quick fix should get you through a gig or two and hopefully solve one of life's minor niggles.