Master Cylinder design change

[RJChallenger]

When did mother Mopar change the shape of the master cylinder from the rounded front to the square front? Are the older style still readily available?Thanks

[Cuda Cody]

I've been pretty lucky over the years finding rebuilt ones at my local parts store.  But those times may be gone now.  Not sure when they changed designs. 

[JH27N0B]

Your question is somewhat vague.  Are you talking about drum or disc MCs?  The '67-'70 drum MCs were sort of square front looking to me with a bolted on cap until they changed the style in '71 to a rounded front with a bale wire retained cap.
Disc Mopar MCs had what is commonly referred to as the "thermometer style" front design from '67-'70 and that changed in '71 to a style where the bore area of the front of the casting protruded out close to 1", except for a rare hemi only thermometer front style MC that carried over through the '71 model year on disc R codes cars.
There's several aftermarket MCs that look somewhat close to OE for cars that aren't going for concours or OE type restorations.  But otherwise you are stuck looking for an original core to restore or a restored or NOS part,  which sadly typically are pricey.

[RJChallenger]

 I'm sorry . It was vague . I'm talking about power disk brake MC. In 70 the small chamber at the front was round, Then sometime afterwards if you ordered a MC for a 70 challenger you got a MC with a small square chamber at the front.

[JH27N0B]

OE were both rounded front chambers, the 70 had the "thermometer" syle front, the '71 had a protruding bore area front.
But I know the aftermarket 71 up disc MC is square not rounded. Raybestos MC36307.
Sadly, the OE disc MC for 70 is a one year only part, and are probably the most sought out and expensive ones.  There isn't an aftermarket MC that looks like it.
The closest appearing aftermarket one is the one for disc brake B bodies because it has the thermometer style front.  It is a 1-1/8" bore vs 1" for the Ebody MC.  It seems to work ok, I had one on my Challenger convertible for a while.  The rear brakes seemed a bit "grabby" but otherwise OK.
It is taller and narrower than an E body MC. Raybestos MC36283.
There is an OE Mopar MC that looks a lot closer and has a 1" bore, and is a lot less money if you find one.  It's for 67-70 A bodies and has a casting 2225541.  I call it a 7/8 scale E body MC.
Otherwise you are stuck seeking a casting 2229171, and prepare to fork out $500-$800 for one.

[Chryco Psycho]

I just went through this back in Aug , I tried to get Any disc brake master cyl with a rounded front , I had them shipped in from all over North America , the catalogs all showed round front master cylinders , I never found Even one all of them have been replaced / substituted with square front , I could get a variety of different size bodies but nothing with the round front . the 70 171 master is approx $1000 used , the later round front are $400 up & still need to be rebuilt