While rebuilding the hydraulic piston assembly on my Grandpa's 1951 Cub, I ran into a problem. Removing the Regulator Valve Seat. After several different failures my cousin came up with this idea.
Materials List:
I purchased a Blackhawk 3/4" Deep Well Impact Socket at Carquest.

3/8" Drive Impact Set at Northern Tool

3/8" Pittsburgh Deep Well Impact Sockets at Harbor Freight
Individual sockets may be available as well.
Method:
I measured the width of the slots of the Regulator Valve Seat with small straight screwdriver. Then I transfered the measurements to the socket and using a bench grinder and some water to keep it cool, I ground it out to fit the valve seat. I finished up by deburring it with a file and some sand paper.
Here are a couple of pictures of the end result.


If anyone knows where to buy a tool like this, please let me know.