the one way valve could be blocked.. you'll find that behind the.. gearbox side.. rear quarter trim. you can use fish tank air hose connectors to connect the bits of pipe together.
it could also be blocked inside the tank its self. you can take the pump out of the tank and clean the gauze the covers the pumps inlet.
i took my tank off and filled it full of bleach to get rid of all organic build-ups inside.
you got to be careful though the rubber seals can split when you put them back together.. if that happens you'll have to get new seals or just silicone seal the pump in.
you could also leave the pumps on the tank, disconnect all the pipes and wires from it, remove it from the car.. fill it with a very light well washed gravel mixed with water and shake it like crazy...
then give it some good flushes out to make sure all the gravel is out.
that should hopefully remove any organic build up on the gauze covering the pump inlet.
my money is on the one way valve behind the trim panel though