I had the same problems as you are having, including the starter run- a- way. As It turns out what is happening is the contacts wear uneven and the plunger is forced to wedge in the bore or that is what was told to me.
Today I got my new contact kit in and I compaired the contacts to a few left over from a turbodiesel Toyota 4x4. I am glad I kept the parts now. as the contacts looked the same as the original Toyota contacts. I also had some bigger contacts than the original ons which had a lot better contact in my Toyota, And I am going to use them in the Cummins as they worked better than the Factory contacts.
I made a mandrel to burnish the contacts to get a good contact. I found out very quickly that you need to install the contacts a flat as I could. I used anti-seize on the contacts retaining bolts threads.
When the brushes wear out I think that there is a company call Helwig that sell brushes by a given size, or you can file down some larger brushes to fit as I have done this several times over the last 20 years or so.
You can take a pocket knife and peal out the seal on the bearings and smear a little grease and put the seal back in. They will run a long time like with a little grease in them.
One last thing, make sure that the starter relay is giving the pull in coil 12 volts . LOL Jim