Loved @Swooshdave's comment... get a second bike for parts!!! Never heard that advice before!
Do research on DOT5 fluid. I have successfully converted all my old british iron to DOT5 without issue! The incompatible with Rubber is BS. The trick is to thoroughly flush the entire system with denatured alcohol before doing the change. There are NO ill effects for DOT5. Costs a bit more, doesn't eat paint, good temp ranges. Can you tell I'm a big fan? (If British hydraulics can work with it , clutch and brakes, it'll work anywhere.)
Lastly, you're confidence in your square seal... as advised several times... put it back together with the old seal... see what happens. My suspicion, since there was no leaking visible, is that it'll probably be ok.
I'll be out of town this week interviewing Probate attorneys in Little Rock. Back this weekend if you need someone to put another set of eyes on it or just in case you need a bit of homebrewed Belgian Tripple. (Don't count the X's on the outside of the keg)