yes there's a necessary breather hole in these old MC caps, and that's one of the main drawbacks of the old system [open to ambient moist air] - the plastic caps are common replacements for the originals that always corrode badly - they weren't brass from the factory, but some of the vendors have [or did have] aluminum ones that don't rot, but whatever type you settle on find a rubber O-ring to use as a gasket to avoid having it seize onto the MC

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics