My vacuum tools have very weak suction. - Bissell 3576 CleanView II Bagless Upright Vacuum

Your filter may be stopped up and not allowing vacuum.
Your discharge may be blocked. In order to have vacuum you have to have a way to discharge the air.
Possibly your motor is weak

I found this info
Why is my vacuum losing suction?
A motor generates power which powers a vacuum which, in turn, creates suction to lift debris. The most common areas of suction loss are within the hose itself, the vacuum bag, clogged rollers, broken vacuum belt or a faulty gasket. All of these problems will lead directly to the vacuum cleaner losing suction.

if you've changed filters and checked for blockages it may need a new suction fan ,,you can download the diagram off the link below
http://www.ereplacementparts.com/black-and-decker-phv1800-dustbuster-parts-c-4167_8887_8897.html

is there a blockage in the inlet pipe ???

Is the tank filled completely with dirt? When was the filter last replaced or cleaned?

Check the motor and clean it properly. A very dirt motor will cause no suction or weak suction.
Be sure the motor is running on proper speed, a weak motor will cause a weak suction.
Thanks for asking.

If you are sure the is no leaks on suction line, please check and clean the filter. The common problem on weak suction vacuum cleaner are leaks and filter clog up.

Regards

Hello,
It sounds as though there is something stuck in the hose or outlet from the motor to the hose.

On the Hoover U6311-900 there is no switch that turns off the suction to the tool hose. That hose is attached to the head of the vacuum on the side via a spring loaded "door". Just twist the hose to remove it, the door will close and you will have full suction again.

clean the filter abovethe dust cup... and any other filters

Either you have clogged filters, or the motor is weak, you could check the carbon brushes, otherwise replace the motor.