Both of these valves on my system have slow leaks. I can find Schrader valves but these don’t look like Schrader, but instead just balls probably with a spring underneath. Most of the stem will screw off, but I doubt if a Schrader valve will fit in.
BTW I replaced the fuel filter (140,000 km) and it seems many of the symptoms such as rough idle, stalling when stopping after a cruise, etc, have disappeared. The pressure difference when blowing through the input end of old and new filter is significant.