The hose can be flushed out using a garden hose. The problem with wet vacs not sucking into the bucket is the float ball sticking. It rises and seals when the bucket has reached its fill level. This stops water reaching the motor.
https://www.youtube.com/watch?v=uLIlPuc4MbERinse the dirty water tank and lid under water (max 40oC) to remove dirt/debris. Clean the filter screen located at the back of the dirty water tank. To re-fit, tilt the dirty water tank and line the front of the tank up onto the carpet washer. Push back until it clicks into place.
http://blog.vax.co.uk/cleaning-maintaining-vax-platinum-power-max-carpet-cleaner/
According to an Amazon web site found using https://www.google.com to search:
1. To Reset the Air Filter Counter, turn the thermostats dial to Air Filter. 2. Press the Up & Down buttons together to reset the Air Filter counter to the default Days limit (90 days).