Master cylinder rebuild

My 1964 Valiant needs a master cylinder rebuild. When I took it out for the summer and checked all levels, I noticed my reservoir was basically empty. Hadn't touched the pedal so refilled. No leakage evidenced at the four corners. The firewall paint has been compromised, though no fluid observed. And the carpet is dry where the pushrod enters. Meant to check since then, but you know..are brakes important?!! So yesterday had a low pedal and turns out my reservoir is dry again. So, I obviously refilled. Long story short (too late!) I'm going to do an old school poor boy rebuild. NAPA has a kit to do so. Anyone done it? I have a hone, I'll let you know how it turns out. I'm a bit concerned with removing the master. Can I leave it in place for the rebuild? Or must it be removed. It's always good to get input from the knowledgeable, versus just winging it and doing a bunch of unnecessary work!

TIA