Classic/Antique Car Repair: low brake fluid, floor situation, brake job

Question
After following your suggested ride exercise, that is top speeds of 70mph, once off the main road and lower speed area,my braking came to a pedal to the floor situation. Quick inspection yielded low brake fluid. No Leaks. Do I just top up, check for air in system ? Next day showed level returning just short of normal. Is this due to the boiling point at high speeds or the prior owner not finishing the brake job of through bleeding? The Rolls Royce is a 1969 coupe. I read where Dot 3 can substitute in a situation such as this, true or false?

Answer
Hi John:
I think you may have communicated with another expert, as
I don't recall the process you describe or solution you describe.
Please send or reply to my original response, so  can see what
the original problem was. In any case, NEVER PUT DOT 3 BRAKE FLUID IN A RR.
IT WILL DESTROY THE ENTIRE BRAKE SYSTEM. ONLY USE RR363 Mineral Oil.