I just finished bleeding my 49 half ton's brakes and when I went to put the fill plug back into the master cylinder, I noticed that a thin "gasket"(?) was dangling from the plug. I am guessing a gasket or some sort of seal is needed to keep the system air tight, but where can these be found? After looking at the usual Chevy truck online part sites I found MC rebuild kits that include a fill plug/cap and an o-ring that looks like it might go into the fill plug/cap, but I have not found just the gasket or o-ring.
Is a seal necessary, or will keeping the plug tight be enough of a seal?
