99% of the time issues with vacuum motors are not the motors, but that the arms and blades. My 66 M35A2 had vacuum wipers and worked great one I replaced the straight arms and blades with spring loaded ones.
Be VERY CAREFUL about shooting goo into the vacuum motor. I'd clean it and lube it properly with light machine oil (3 in 1 is a good type, or sewing machine oil) as this does not gum up over time.
The pot metal housings do have wear points, and they can leak causing poor operation.
