The valves are check valves, both work in the same flow direction, I think you should be able to blow form the tank line but not back (obviously), unless something is backwards and I don't know if that can be done but looks like one is backwards, I will say you need a new pump, but before that, double check( I know again) and try blow from the hose with the pump disassembled so you can identify the ports and then visualize the way everything should be.

Make sure you reassemble to pump with new o rings and seals.

http://www.yspower.co.jp/pdf/DZ160-E.pdf It's easier to zoom in the pump than the actual manual.

This is a very good article on how everything works, don't know if read it before, but just in case.


http://www.gbrcaa.org/acrobatfiles/Y...%20Systems.pdf