i have an 82 z/28 that came standard with disc front / drum back,,,,, car has a new brake booster fitted..

put a good 383 in the car and struggled with brakes stopping at the end of the 1/4.mile strip..

done a c5 corvette front brake upgrade,, now the master cylinder needs a bit more pressure to compinsate the larger front callipers..

is there a 1 1/8 piston diameter master cylinder available that will go straight on without too much playing around
i need more pressure end of story.,.
i can run a seperate adjustable front rear valve to dial it in..

I have an 82 TA and have wilwood 6 piston 13inch fronts and 4 piston 12 inch rears. I have a new but stock 4 wheel disk master for an 82.I used a wilwood prop valve. It stops awesome. Like really fast. I'd try the prop valve first.

So what is the actual behavior of the car that's undesirable? Just a description of the behavior, please. No interpretations, assumptions, theories because somebody else [more experienced] might not see it that way.

And what rear brakes do you have? And what proportioning valve? And what tire sizes and types do you run front and rear? (Is this a drag car with soft springs and drag shocks?)

Regarding the master cylinder, a 1-1/8" bore is larger than stock, so you would end up with even less line pressure for the same pedal effort, and the pedal will feel harder with a shorter throw. It might appear to be 1.25" bore diameter from the outside, but what you're looking at is the larger quick take-up chamber that is not part of the high-pressure system. The actual high-pressure chambers are something around 24 mm diameter (if I remember right). Getting a car to stop quick is about achieving proper BRAKE BIAS front and rear. The brake master cylinder usually has nothing to do with this.

Watch this slide show if you want to learn more about the quick take-up function, https://slideplayer.com/slide/4616100/
Explanation begins at about 17 minutes, but it is helpful to start at 13 minutes so you can build on the concepts to understand.

i drive the car on a back road to test the brakes,, gut to 60 mph ,, slam the brakes on ,, car will not stop dead or even lock up the fronts,

ANOTHER example

i have a 1993 GMC C1500 2x4 350tbi auto pickup.
bought it and noticed the pedal allmost goes to the floor then the vehicle stops slowly.
i have no idea how these vehicles passed safety regulations when new., all the brake components are new,, tires and shocks new.

i saw on youtube a cadillac non ABS master cylinder is a common upgrade ,, larger piston in cylinder..
bought one, put it on, car stops with 1/4 the pedal travel, stops fast, is no longer an accident waiting to happen, it doesnt feel goofy or dangerous to drive, now stops like a modern car and not a worn out farm work vehicle.

i bought 2 of these cadillac master cylinders hopeing i can put one on the camaro,,
but the pipes come our the wrong side, the resevior is too high cant shut the hood.

the stock camaro 1982 has under 1200psi at the calliper i have been advised.

there is a company in America called ABS brakes, they sell an electric unit that does away with the stock booster and master cyl and uses an electric unit but there $2500
they run 2450psi at the calliper and require very little left leg to get there.

i am in Australia and cant go to junk yards buying parts to try..

This is the only useful part of your post. And you didn't answer most my questions.

Is the master cylinder on the 82 different than say, the 88 or 89 master? I ask because I've done the LS1 front brake upgrade on both of my Irocs and it was a night and day improvement without any master cylinder change, and that's before fine tuning the bias front and back. The c5 brakes should be very similar I'd imagine. Is it possible there's something else wrong?

The 82-83 masters are a different part no due to the the lines having different fittings. They have a 24-25 mm bore and are perfect for these setups. Mine has tons of brake power but maybe a little more travel than I like (personal preference) but pedal effort is minimal. I'm assuming there's air in the lines or the MC is shot.