[Spridgets] brake fluid question

[ryan.kubanoff at comcast.net]

I have a 74 with a bad master cylinder thats going to be replaced tomorrow. But now the question is brake fluid and what to use. I presume that the system has original or older DOT4 although last month when i noticed the leak in the resevoir i topped it off with 20ml of DOT3 (i didn't know any better). Today when I went to pepboys for a quart of DOT4 all they have is the castrol synthetic slm that "meets or exceeds DOT3 and DOT4 standards". now i have not seen any more leaking in the past 2 months so i am thinking that my DOT3 addition is still in the master resvoir but? 

Advice needed. do i drain the whole system and refill with the new castol stuff, do i drain just the master and refill with the new castrol, or some other suggestion. 
Thanks 
ryan