I have rebuilt and restored a number of master cylinders. It helped that I used to work for a brake part manufacturer!
There are kits available for many master cylinders, but not for others. Are you trying to rebuild the master for the '70 in your avatar? Is it a disc or a drum brake car and is it the original master cylinder? You mention black paint, so I am guessing you have a disc.
You list a '71 Challenger in your bio, those use a 1-1/32" kit that should be easy to find at NAPA or other parts stores.
There is no kit available currently for the '70 disc brake master cylinder that I am aware of. This MC can be identified by the casting number on the bottom which is 2229171. This is what I have on my convertible and on my T/A so I know it well.
You might be able to work with your local parts store to find kits for different '70 disc brake cars and pirate the seals from those kits to rebuild the pistons on yours. When I worked at the brake company I was able to dig through samples until I found correct seals to rebuild mine.
I first media blast my casting after disassembling it, then I clean up the bore using a cylinder hone I bought from a tool vendor at a swap meet, they are easy to find. You mount them in a drill and run it in and out of the bore, after lubricating with a little brake fluid, to clean it up.
If your bore is pitted, you are probably not going to be successful. If you hone it enough to get the pits out you will have increased the bore diameter enough that you will get leaking or maybe even no braking when pushing the pedal because the seals won't hold pressure!
Therefore if you have pitting or a bore that has been honed to too large a diameter, the cylinder must be sent in to be sleeved with a stainless steel or brass sleeve.
Next you replace the brass seats from the outlets with new ones that come in the kits. The kits contain screws that can be used to pull the old seats out of the outlets. You can place the new ones in the holes and push them into place by carefully tightening in some brake line. The brass is soft and you don't want to distort the brass too much until you are installing the MC on the car and tightening the brake lines during your installation.
After replacing the pistons with new ones from a kit, or by carefully replacing the old seals on the pistons (and making sure they face the correct way!), install the pistons in the bore. It is best to prebleed the master cylinder on the bench before installing on the car, this will save you a ton of time and effort.
As far as painting, brake fluid will certainly remove the paint. DOT 5 is nice if you use it as it will not damage the paint.
However if you use DOT 3 or 4, and your cap seal is tight, all is not lost. When prebleeding and installing the master cylinder, wipe any fluid that spills off immediately before it has a chance to damage the paint. I have touched up minor damage with a spray can and it looks nice.
You can send your master cylinder off to be rebuilt. I use Brake and Equipment Warehouse up in Minneapolis.
Here is Brake and Equipment warehouse's website:
http://www.brakeplace.com/They are great to work with, if you do, tell them Brad who used to work at Brake Parts sent you!
