[FortyNiner]

Some of the master cylinder repop units (including the NAPA version) have the dumbest design possible for the cap/throat. Impossible to add fluid without making a mess, they compound the fun by preventing any means to guage the fill level. So, you can't assure adequate fluid and any attempt to top things off is rewarded with a puddle on the floor.

Fortunately, the old, simple, foolproof screw cap fits - replace the 'new' one with it and live happier.