In hopes I could avoid having to remove my stick and crawl up under my
panel to reach the tops of the master cylinders, I called Matco Tech
Support.  This only applies to the MC-5 (internal reservoir) master
cylinders.  Turns out filling the system from the bleeders is the
preferred way of doing it.  The opening at the top of the master
cylinder where the piston shaft comes out is not air/fluid tight (main
reason for the rubber booty), so it is not necessary to remove the
filler plug at the top of the reservoir to vent.  However, Matco
recommends that the reservoir not be filled to the top but rather no
more than 1/4" from the top (thus requiring removal of the plug to
measure the fluid level!).  This is to allow for expansion and to avoid
leakage from the shaft opening.  So you guys who are filling the
reservoir 'til it oozes out are technically overfilling it.  But Matco
says it doesn't cause any harm, just mess.

I think I'm going to opt for filling from the bleeders until I see fluid
just starting to appear around the shaft opening (A&P pumping, me
watching).  I can reach the top of the master cylinders with a rag
without removing the stick if I need to dab up the little bit that gets
out.  (Anybody wanna bet we end up spraying fluid all over the bottom of
the header tank?)

Lee