the design has been changed. they used to have 5 balls in, this has now been reduced to 4 balls, this makes the internal valve less stable. so more balls the better. i've got a supplyer that gets me the 5 ball variety and they REALLY do last longer. i always carry spares of both M&F and change both at the same time, i've had a 4 ball type fail after 1 use, now THAT'S annoying.
Dave