how can i fix the seal on my vax ?

my vax when in the wet mode, is leaking water from the motor seal. there is suction but not much and the ping pong ball that stops the water going from the bottom cannister to the top can't be heard dropping down anymore. when the motor part is removed from the cannisters it has awesome suction. the water is then coming out from the motor seal and dribbling down near the suction pipe.