Master Cylinder diaphragm leaking / 1970 pdb

[JS23U]

My top seal / diaphragm on the disc brake mc of my 1970 Challenger is leaking. It seems it does this in stop and go, when the engine bay gets hot. Or maybe because the brake fluid gets hot from braking in stop and go...
Anyway, I already had bought an old aftermarket diaphragm (with a 1986 date) and it didn't really help. Maybe the NOS part was too hard already? Are these diaphragms still manufactured? I couldn't find any. I have a MC with the rounded front, not square like the newer ones. Therefore the newer diaphragms don't fit. The top surface of the casting looks good, the fluid is changed, I tried a new cap and bail, nothing seems to help. It got a little better but is not gone. Any ideas?
If I can't get rid of that problem I am thinking about replacing the MC with an aftermarket one that looks stock. I am aware that the shape of the casting is different (square), see above, but my car is a driver. Which would be a correct replacement function-wise? I found a Raybestos MC 36283 with a different diameter (1.125" vs. 1"). Will that one work on my car plug&play? If not, which one would be a better choice?

See the current mc attached. The casting doesn't look like a 1970, does it?

Sorry, much text. Thanks for any input!
Jens

[MoparCarGuy]

What is the casting number on your MC?
It appears to be a 1971-1975 Chrysler Part #3461176 (Casting 3461187) which looks more correct than any square-front MCs and is one of the best for power brake cars.
You may want to call The Ram Man about your issue and also match the MC gasket to one of these if he says that is your problem: https://therammaninc.com/products/shop_list/Master-Cylinder-Gaskets-439

FYI - The correct 1970-only Power Brake MC is Chrysler Part #2944453 (Casting 2229171) for E-Body except Hemis.

[JS23U]

Thanks for the reply, MoparCarGuy. I will try to get a photo of the part number next time I am in the garage. It really looks like a 71 and up piece. The car probably got a replacement while in daily service in the 70s.
The Ram Man seems to have a lot of hard to find stuff, I didn't know that site. But if I understand it right he sells used diaphragms or maybe NOS ones. I'm not sure if that will help over the NOS part that I already bought and used.

[Bullitt-]

  I wonder if the bore difference is related to a previous MC replacement.
They list a larger bore for '70-1.125 In than '73-1.031 In
https://www.stockwiseauto.com/dorman-m71297-brake-master-cylinder?Year=1970&Make=Dodge&Model=Challenger
  https://www.stockwiseauto.com/dorman-m75818-brake-master-cylinder?Year=1973&Make=Dodge&Model=Challenger

[JS23U]

I just took a photo of the MC in the Challenger. It is indeed the 1971 and up casting 3461187. That means I already have a MC with a bigger than stock (for 1970) diameter bore. Obviously it works, assuming that my MC hadn't been modified in any kind. Not knowing what it looks like inside, and as it is not the correct one anyway I'll probably look for a replacement 1971 style. I hate that leaky one I have now, want to get rid of that problem.

But I still wonder why they changed the diameter at all from 1970 to 1971. Isn't the rest of the PDB brake system all the same between 70 and 71?

BTW, my Challenger is late June 1970 built. Is it possible that this MC is original to my car?