The post filter sits behind the motor - air passes through the motor before reaching the post filter.
If the post filter is clogging up the motor will have had a good dose of dust and grit and this will have very probably shortened its life.
Your challenge is to discover why the dust cloud is not being stopped by the pre filter - why it's allowed to pass through the motor - clearly there's an assembly problem
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix vacuum cleaners.