This trick is basically for cleaning the pipe and works especially well on a vertical length: Take a small ballpeen hammer and tap the pipe about a dozen times above your cleanout T. Now ya don't need to smack it so hard that you leave a mark, just some good firm taps and be prepared because loads of ash will drop. I usually remove the cleanout T cap, position the vac nozzle underneath, then start tapping on as much of the vertical vent as you can reach. Follow up by running a brush up the vent a few times, but the tapping easily removes 80% of the crud. I've got 15 ft of vertical pipe running up inside the chimney and this method works awesome. Been cleaning it this way since day one.