Starter problem

The starter on my 67 with a f-134 is giving me trouble. When I went to turn over the jeep in prep of starting it after a resto, the starter just hummed kind of and didn't crank the motor. Thinking I had too much paint on it preventing a good ground I pulled the starter off and hit the flange and the bell housing with a 3M whizz wheel. While I had it out I put the starter in the vise and jumped a battery to it. I got the same sound it made in the jeep. the starter was turning but not fast enough. So I pulled the end cover and douched it out with brake clean and WD-40. After a bunch of black nasty stuff came out of it the brushes and comutator looked good. I put a tab of oil on the bushing and reassmebed it. I tried jumping it again in the vise and it spun much faster and the bendix moved forward. I bolted it in the Jeep and tried again. I did turn the motor, but only maybe a 1/3 of a rotation. This was after the started spun for a second before engaging the flywheel. It seams to disengage and continue to spin after turning the motor only 1/3-1/2 of a rotation. Any ideas? anyone here have a similar problem?

Thanks!