When the valve is becoming hard to
press inside, inflexible when opened or the stopper is becoming too
small to fit properly there is the time for to let to soak a new
plasticizer inside. Using paintbrush apply thin layer from the outside
of the closed valve and let it soak for about two or three days. It
works magically, the body of the valve is much softer again and the
stopper is bigger so the valve safer stays closed.