This is a general comment on air filters, so it may be no good in your case. Depending on the fragility of the filter, you can often vacuum accumulated dust right off the filter using the hose attachment without any attached brush or nozzle. Always do this from the incoming side of the filter to avoid sucking dust through the filter!
I have a Panasonic room air filter and vacuuming the prefilter is something they tell you to do in the manual.
If you're going to try this on a filter that doesn't look very tough, just adjust the distance you hold the hose from the filter to suit.
