I have a Hall Toledo valve seat grinder and have used it thousands of times. I bought stones from HT about 10 years ago and as far as I know, they are still in business.
A few pointers I will suggest is to be sure to clean the valve guide so the mandrel will tighten properly. Using the tool to insert the mandrel, push it down into the valve guide and rotate it as you go down. You will feel it get a little tight. Once it is all the way in, tighten the drawbar, but just snug, do NOT horse on it!
If you over tighten the mandrels the drawbars will break easily. I bought a new 14mm mandrel a few years ago as it is, or was the most common and it finally broke the drawbar shaft. The first no-no is do not use the automatic feed, it is too fast and will ruin your stone. Feed it by hand and learn the sound of the stone contacting the seat.
Always true up your stone, but you don't have to overdue it, just make sure you see the diamond go across the stone all the way. When the stones get thin, be careful when you are truing them, they will explode!
As far as putting anything on the seat to see where it is grinding, that is a waste of time. When it rotates on its orbit you will see the sparks where the seat is high. Eventually you will see sparks come out all the way around the orbit. The seat is very shiny, there is no need to put anything on the seat, plus it is easy to just raise up the grinder and look.
You will eventually get a nice shiny seat all the way around. You can use a flatter degree stone to narrow the seat or use a narrowing tool. It fits on the mandrel and you turn it with a ratchet.
Mine is the second one I have owned. The first one my Father bought and it came in a black box. We finally wore out the orbital carriage and you had to hold the motor up or it would not grind true. The new one came in a green case and I still have it, albeit I am retired so I don't get much opportunity to use it. I certainly don't advertise work as I like being retired without any obligations except to work in my shop on my own projects. Any questions I can answer, feel free to ask. As I said, I have worn out one machine and done thousands of seats on my newer machine.
As far as fixing the old machine, it was not much more to buy a new machine than to fix the old one. One more thing, Hall advertised the machine to be so true you didn't have to lap the valves. Their demonstration was to grind a seat and compare it to a lapped seat. The ground seat held a vacuum better than the lapped seat. They are, as the gentleman above said. the best seat grinder ever made!
