If you have an original master cylinder with the bladders, to a point it might be sealed, at least until you lose a large amount of fluid. All newer vehicles have a "venting slit" in the cap which will open under pressure or vacuum. The 80s aluminum Mopar Master cylinders have this feature too.
Depending on the fluid it usually says on the can 2 or 3 years. My opinion is you put DOT4 in everything that started with DOT3 since they are compatible. Anything with ABS should get DOT4 LV ( ATE SL.6 or similar) and our old cars should get regular DOT4 ( ATE Typ.200 or similar). If you have really dark fluid or rust in the master cylinder, I would suggest not being too aggressive when pumping fluid through, because in some cases it will wreck the master cylinder due to all the junk that was in there getting caught in the seals.
Don't put DOT5 anywhere near that already filled system, asking for trouble, and it's also more compressible so it decreases your braking performance.
