Waste posted this in another thread and I'd hate to see this excellent tidbit get lost
If you have a separate pump and filter which doesn't have a multiport -
Remove the filter element, if possible, and open the drain plug - attach a hose to it if you don't want to flood the filter pad area. Put plugs in any return ports, if you don't have a valve to kill the returns. Vacuum through the, now empty, filter.
Should be very helpful for those with cartridge or non-multi port filters