does anyone know what the piston diameter is on these stock master cylinders?

i have the traditional stock style hand controls on my 67 that i want to get rid of.

i have dual jaybrake 4piston front calipers, they work fine right now so i want to make sure the new master cylinder i get is the same size

ive searched google and part numbers without any luck. thanks in advance

 

Not sure but based on some googling my educated guess would be 3/4in bore

 

Stock is 5 /8 inch and uses dot 3 brake fluid

you can buy a dot 5 kit for it

but 4 piston J brake requires a dot 5 and maybe a 11 /16 master - call j brake - know the rotor diameter as well

 

Ran dual piston dual calipers with the stock old style shovel MC before, while it works the level travels a lot more and you just know there's more there than you are getting. I currently run dual 4 piston Wilwood calipers with the 82 and later style 3/4" bore MC, bike damned near do a stoppie if you grab to hard a handful now.

Not a big JayBrake fan anymore. While they were the **** back in the day current cheaper alternatives are much better calipers performance wise. This is my rear setup, the pads alone will run $90 to replace come, only did this because a supplier I was using gave these to me to $50 each new and I was bored one winter.

 

i dont think theres anyone to call now. last i read supertrapp bought them in 2011 and nothing on their website shows it. i found one old PDF catalog from 2015. i think youre right on the 11/16

 

I always thought 11/16 or even 3/4...and all of mine from mid '70's up have called for DOT 5..
The Old master cylinders were the MoCo solution for anti lock brakes...took a gorilla to lock up a front wheel with the 3/4 and a Pie Slice...put a 5/8 on and ...Wow... Power Brakes....