Master cylinder lid seal 68 dart

Common, and why every A-body I've seen in the junkyard (or bought) had a rusty firewall under the MC (great paint remover). The cap needs to be airtight to keep moisture from the air being absorbed in the glycol fluid (DOT 3, 4, 5.1) to cause internal corrosion. Less concern w/ silicone (DOT 5), but wouldn't want that expensive fluid sloshing out.

Even with a good cap, you should flush thru new fluid say every 4 years in eastern U.S. (maybe 10 yrs in west). Newer MC's have a flexible rubber diaphragm to allow volume changes without sucking in outside air. Most old caps just have a circumferential gasket, so perhaps need to exchange outside air as the fluid level changes (to not draw a vacuum). Many people toss the rusty cast-iron MC for a newish alum one like for a 1980's Dodge truck. Those are 2-bolt so need a 2 to 4 bolt adapter plate. Can buy on ebay, or Dr Diff sells an MC w/ plate.