That's true for any ...
"....
always clean the brush roll after you're finished using the..."
power nozzle, or for that matter any brushed attachment like a floor or dusting brush. It's like this: when one uses a paint brush, tooth brush, vegetable brush, etc. you clean those (or should be). The same goes for a vacuum brush. Failure to remove old hair and string only inhibits a vacuum brush and can damage the roll or possibly the power head.
Here's some pics of my reconditioned 1983ish Kenmore double-brush power nozzle. I wish I had the "before" pictures. I got it with some other vacuum parts and I don't think the seller even knew what it was.
It looked really bad. It had a big spot of paint dripped on the top, was dirty from sitting many years, the neck was cracked (though oddly still useable), the cord was pulled out, there was so much sand and lint in the motor it made a horrendous sound, and the brush roll was just covered with string, a couple of pieces of thin guage wire, hair, and who knows what else.
After thoroughly taking it apart, soaking plastic parts and carefully cleaning, removing all the gunk from the motor and lubricating it, and of course taking a scissors and cutting length-wise and removing all the junk on the brush roll, removing and cleaning the brush strips, etc. . it now functions and sounds as it should. The brush roll is black painted wood, which was scrubbed and waxed; but look at the paint worn off by the stuff that was on there.
It still bares the scars of damage done. How in the world could someone not know there was something wrong with what they were doing? Look at the gouges in the plastic underneath. Thank-fully, those don't affect cleaning or suction. Incredibly, it still has both of the thin plastic support bars that are known to break.
Always clean your brushed attachments after each use. It only takes a few seconds with the wand of the hose going back and forth over the brushes. And a brush roll with string or long hair wrapped around it will often clean itself if you simply cut with a scissors, length-wise through the mess, and then vacuum a few strokes on carpeting. The brush roll can throw off the cut pieces and suction takes it away.
I love taking stuff like this and bringing it back to full functionality, or as close as it can get.
