1940 Master Cylinder Rebuild ??
 

I have a 1940 master cylinder (casting says Wagner Brakes 1 1/16 ) that the bore is perfect but has been sitting a long time and decided to put rebuild kit in it.
I have 2 rebuild kits - one came from NAPA and the other from aftermarket parts supplier.
Attached is a picture with the MC piston and the rebuild kit parts as follows:

Left most - original piston from the Wagner - has a thin metal disk riveted to the piston pressure end covering the 6 small relief holes(not seen).
Center - a vintage Ford aftermarket supply parts source - has 6 small holes in piston
Right - local NAPA source - has the 3 larger holes
The pistons were virtually identical with only minor casting differences.

I also noticed the return spring in the aftermarket kit was about 1/2 longer and decided to use it. (The NAPA kit did not have a spring.)
So I decided to replace the rubber cup on the original piston along with the other renewable parts.

I did notice the NAPA kit is USA made and the aftermarket kit was from China and was also much more expensive! I guess Chinese springs are expensive - ha ha.
Subjectively, the rubber pieces in the NAPA kit are much higher quality.
I did determine Dorman makes these kits as well and generally lower priced as well but haven't seen the components.

Anyone aware of why the MC piston pressure holes are different between the kits and if one design versus the other is preferable?

Re: 1940 Master Cylinder Rebuild ??
 

It it cheaper to drill 3 holes rather than 6 holes.
No other explanation.

Re: 1940 Master Cylinder Rebuild ??
 

The thin metal plate covering holes on the original piston is what struck me as odd.
It looked like it was thin enough to potentially act as a reed valve.
I assumed 3 large probably has a similar surface area as 6 small on the 2 replacements.