Ok so I have a datsun sunny b310 sedan with a ca18det. I am getting an r31 diff cut down and i have s13 brakes on the front im a little confused on how to determine about brake booster master and portioning valve.

Use the B310 disc brake master. B310 also comes standard with automatic proportioning valve.

You sure my engineering guy said i need to upgrade it to compensate for bigger brakes

B310 brake master 13/16"
S13 Silvia master cyl 13/16
R31 master cylinder 7/8

So if you went to a 7/8 master you'd be more than safe in theory
JB1133 master cyl is 7/8 twin bottle and should do the job
Just remove the residual valve to the rear brakes so the rear discs don't hold on

Its about what your engineer requires, so best to ask him. Some guys here use stockers with success. The larger masters requires more pedal effort, though some guys like that.

I passed engineers pedal force and repeat brake tests with 11/16th and no booster. Hilux 4 spots on front and skyline discs rear. Engineer was not keen cause he thought too small but said he was willing to see if it would pass cause I had already been running the setup for a couple of years and also run similar on the rally car.

Thanks guys :) ill do that this week hopefully