So far I cant get the secondary piston out of my 409 master cylinder......

What fluid should I pouring into bore to loosen things up ????

I would use WD40 and let it soak. You probably have to re-insert the primary spring assembly back in and hit it a few times to help dislodge the secondary spring assembly.

Once removed your going to have to access the condition of the bore. They can be honed to remove surface defects but it there is pitting or other scoring going on not sure I would rebuild it as it just may leak past the new seals on the rebuild kit.

I was able to drill hole in primary piston and extract with slide hammer....

Any advice on how to drill into and pull out stuck secondary piston

Was finally able to find a tool that could grab onto nipple end of secondary piston and remove...After cleaning ran a 3 Bladed Hone thru bore.....Minimal Pitting that I believe
will be Ok...

Now looking for 1970 MC Rebuild KIt that contains BOTH Pistons and Rubbers...
Measures 1 1/8" Bore

All kits Ive looked at Dont list 1970 application ????

https://www.classicindustries.com/product/mk501.html

Try Rock Auto!! I was able to get a rebuild kit thru them for my 70 Nova, which came with everything in the kit including pistons and seals.

I could not find MC Rebuild Kits at RockAuto.com......I saw several brands of Aftermarket
Complete MC's....

Am I to buy a complete unit and xfer the guts into my 1970 "409" casting, EB stamped Housing ?????

Try Ken at Lonestar Caliper Co. He sells rebuild kits.

Seeking info on 409 MC stamped "CF"

Looks like Chris W answered my question in post #2......I have a "409" BE master cylinder and bought it in 2004 wanting a 70' Camaro "409" EB