They (somebody) make(s) seat re-conditioning tools. It consists of a handle that threads into the valve body and has a reamer on the end for re-shaping the seat. I have one at home but I'll be danged if I know where it came from...

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250