For me I would look for a new master cylinder. I`m kinda funny about brakes since you need them to stop every time.

First off, are you sure your master cylinder is the original one? The 70 E body disc brake master cylinder is a one year only, E body only part, and is rare and valuable.
You can identify it by the casting number on the bottom, 2229171.
It's a 1" bore and there is no rebuild kit available for it, at least not available for the last 30 or so years unless you can find NOS. There's a kit for the 67-70 A body disc brake master cylinder, but the piston is too short. You can carefully remove the seals though, and use them to rebuild the original pistons from a 2229171.
There is no aftermarket replacement master cylinder that is accurate for a 70 E body disc brake. I've used Raybestos MC36283 which is a B body master cylinder and has a 1-1/8" bore. I've also used the master cylinder for 71-74 disc brake E bodies, MC36307, and it works OK too.
The Mopar drum brake master cylinder from that era has a 1" bore, but don't use it on a disc brake car!
Brake and Equipment Warehouse in MN is my recommendation to sleeve and rebuild 2229171s.

Here's the link to Brake and Equipment Warehouse.. LINK

whitepost restores them

Yes it's an original. Here's a link to the reproduction one I mention. Have to scroll down to part BR-51511R. Looks like this.

https://www.jimsautoparts.com/brake.htm

From that top view you can't see the front, but it is apparent from the picture that the bore area of the casting protrudes, whereas the original 70 disc brake E body master cylinder is flat. Some call the style of the front of the '70 thermometer shaped. See picture attached.
The one in that picture looks like the 71-74 style, which works fine but doesn't look OE on a '70. It has a 1-1/32" bore.
I'm sure others will chime in about Jim's Auto Parts.
Avoid White Post on 2229171s. They have used the wrong kit with too short a piston, then the master cylinder doesn't work, and according to several members here who had them rebuild their master cylinders, when they called to complain they were told to go pound sand.

Good addit'l info! #1 find out if yours is original. if so then open it up & clean it then visual to see if it appears to be rebuildable with honing and a kit (if as said you can find a kit, try eBay also) & I highly recommend the flexhone. if you have to send it out find a reputeable business to sleeve it & I always remembered what was posted years ago on jims and jacks auto parts: J-I-M = B-A-D and J-A-C-K = G-O-O-D. If yours ain't original & it sounds like it ain't then I'd get a parts house rebuild of the same bore dia (is your pedal effort/travel OK right now?) & bleed it out & be up & running in no time.

Here's a pic of mine. Galen report for my car says its original. My point with the one from Jims is that it's the closest in appearance to an original that I can find. Everything else is flat and square on the front and lid or completely different looking with a front bottom that protrudes out. Pedal effort is good. Never would have suspected anything from the way it functions. Peeling paint is what got me.

You have the correct OEM master cylinder on your car. I would certainly recommend rebuilding it rather than replacing it, as it is a rare and valuable piece.
I personally feel the Raybestos MC36283 is the closest in appearance to the 2229171 as it has the "thermometer" front on it. It is taller and narrower than the original. It has a 1-1/8" bore. I used one of those on my Challenger convertible for a few years until replacing it with a sleeved rebuilt 2229171, and it worked OK. Pedal a little firmer than OE and the rear brakes seemed a little grabby under hard braking was the difference I noted.
The one you posted a picture of from Jim's looks like a rebuilt 71-74 disc brake one.

The one you saw on Jim's is just a picture , there is no guarantee that what you get will be even remotely correct , and good luck trying to return it or get any satisfaction after said Master shows up.

The fact that you asked this question AND think that there would be a gasket there would lead one to believe this is a little above your pay grade.

Unless the seals are trashed in that master putting new guts in it I doubt is going to fix the leak.

Brake fluid attracts water, the water sinks to the bottom and ends up in the bore and over time rusts the bore. Sleeving it is the only option and it's not going to be cheap.

You also need to check your booster as it may now have brake fluid inside it too.

STAY VERY VERY FAR AWAY FROM JIM'S. The stuff he sells isn't what it is advertised to be and then he won't take it back when you find out it's wrong. A number of members here have had run ins with him before he got outed. Now we all know better and try to keep others from making a big mistake.

Not only what this member said and others, for $259.95, Jim can keep it.

The fact that you asked this question AND think that there would be a gasket there would lead one to believe this is a little above your pay grade.

Unless the seals are trashed in that master putting new guts in it I doubt is going to fix the leak.

Brake fluid attracts water, the water sinks to the bottom and ends up in the bore and over time rusts the bore. Sleeving it is the only option and it's not going to be cheap.

You also need to check your booster as it may now have brake fluid inside it too. [/quote]

I want to remove the original leaking master cyl and replace with a new one. Is it as simple as removing fluid from the chambers, removing lines and 4 master cyl to booster bolts or is there anything else I need to do or be aware of?

new master needs to be bench bled, check the engagement depth of the booster rod into the master piston bore. remove any residual fluid from the booster chamber and bleed the 4 corners.

Sounds like you've been over thinking the problem, which I find myself doing with regularity. You've gotten some good input here on what to do and what to avoid, although some of it was a bit more strongly worded than necessary.

From the photo it looks like you've got a nice car there and you definitely want to get the leaking master cylinder off the car before it does any more harm. Even if you have to put a non-OEM looking master cylinder on for now, you do need to get it done.

I believe the recommendation from the forum members is you take the car to a shop or to someone who does this professionally to have the work done. Unless you've done a fair amount of this type of work before, you may find it more headache than you would like, and you sure don't want to take a chance on brakes.

Save the original master cylinder and you'll then have the luxury of deciding how you want to have it restored, while you're still able to safely enjoy the car.

Remember there are no dumb questions. The only dumb thing we can do is not ask. There's lots of expertise here and I've found the vast majority of it very helpful over the years. Good luck!

After everything that has been advised and it seems that you haven't learned how to work on theses cars yet, please take it to a shop and have a new one installed as suggested, then have the original rebuilt and then reinstall it. Now while they work on the car watch everything they do and ask questions so you can understand what they did and why they did it, then you will understand what we have been talking about. Not trying to be mean, but it's apparent that you haven't done this stuff before and we would hate to see you get hurt and wreck the car. Everyone of us started not knowing anything, but we learned. As far as putting a master on till you get the other one rebuilt just go to a regular parts store to get one for a 71 cause it will be easer to do and cheaper.

I have a new master cylinder that I will install and have my original rebuilt. I'm not taking it to someone else. Every time I take one of my cars to someone else they screw something up and are not near as careful and meticulous as I am. I've done quite a bit of brake work in the past although not on one of these. I can't imagine it being much different, but I do want to be sure on this as it is a nice car so I want to know all the ins and outs before I dig in. Chastise me all you want, tell me I don't know what I'm doing (I don't but will learn), tell me take it to someone else (I won't) etc., but I'll still ask questions here or elsewhere and do research until I have a thorough enough understanding to get the job done and done right.

Brakes are not an area you want to learn OJT, or worse yet find out the hard way you missed something.

IF you do not have a factory service manual get one, at the least.