The retrofit kit from Interdynamics has two low side fittings one with external threads and one with internal threads. The one with external threads requires the shrader valve to be removed, the other one does not.
Thanks Larry for your input on this question, I will see if I can locate the shrader valve tool.
1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T