How to fix loss of suction on a dyson dc01?

Help please. Just got a Dyson DC01. The tube section seems to have a loss of suction, even after everything has been cleaned out. I've taped up the lower rubber valves, can't see any holes in the tube and stripped it down to the motor; which visually looks fine. Only thing I haven't done is change the filters, but I have hoovered them out. Any help would be appreciated. Thank you.