VPI 16.5 vacuum cleaner

I have to ask, have you removed the clamp from the drain tube?

Thanks.

Yes.

I have sent a mail to VPI for advice.

Thanks.

Mine is about 10 years old and I've never had any problems with draining the fluid.  After draining I regularly flush mine out further with water.  It sounds like the drain going out to the hose might be clogged.  Might want to check again with VPI to see what their recommendation is if the drain is clogged. 

Sounds like something clogging the outflow of the tank.  Might gently flush a little water back up the tube to see if you can dislodge the obstruction.