I have removed the seized up master cyl. from my 1954 S I, 86", I purchased a working second hand master cyl., but it is larger than the one that I removed.
The inside dia. of the original M/C (if it is original) is around 1/2" dia. cyl. and rubber, the rubbers in the wheel cylinders are 1 1/4".
The rubbers in the second hand M/C I purchased are 1" in dia., this gives a ratio of 1 1/4 : 1 which without power assist would make for some poor braking (I believe).
The smaller M/C with the 1/2" dia. would give a much more powerful brake assist.
I have no problem with returning the larger M/C and getting a replacement, but I was wondering if anyone else is running the larger M/C (1") with the 1 1/4" wheel cylinders, how do they perform are they alright for the job, any advice would be helpful, Regards Frank.