I am running R31 rears and 280mm s13's on the front along with a 7/8th master with booster and wilwood proportinoning valve on my 1200 ute also.
Should have it back on the road next week for testing. I'll keep you posted how it fairs for comparison

It is something that I can try, I have the front fairly soft but it isn't diving excessively. I'm running S13 coil-over struts so I can increase the dampening on the front to try to see if it makes a difference. I also have heavier springs I can try.
SR20DET brakes will be put on as well early next week. rebuilding calipers today / tomorrow.
I'll try the front tonight and see how I go.

Yeah along with unfamilia's suggestion, is your front suspension too soft? If there aren't problems with the brake setup, your weight distribution would be the next thing to check. Try loading up the tray with a heap of junk and see if it changes anything.

Any chance it's nose diving at front under braking and unloading weight over the rears causing them to lock up earlier

Other than bigger pistons in the front calipers your other option is smaller pistons in the rear.

Are they rear drums or rear discs? My old Astra had worn rear discs which had a tendency to grab and lock. All fixed with new rotors and correct adjustment of the handbrake. Drums can grab if the hinge points aren't right, I've never had a problem like that with them though so I'd just check it's all assembled correctly.

Dunno if that helps. Good luck!

It's also a ute which doesn't help with weight distribution.

My understanding was that the master cylinder will stop applying rear brake force at a certain travel and that different master cylinders have different points at which this occurs. I'm not saying that's right but that's how I understood it. Some also have internal proportioning like you mentioned.
As for tyres, I tried brake tests with the front runners on and still had the rears lock. I also tried with similarly matched 14" wheels on brand new tyres, same result.
Brakes work well enough, passed the force test, but rears are locking before fronts.

Several club members have S13 250mm front brakes and R31 Skyline rear brakes. I don't recall any reports of rear lockups

I'm not sure you can judge it by whether the front tyres lock up, as that depends, for example if you've got large tyres they may grip too much to lock up

The important thing is that when doing panic braking, that the rears don't lock up before the fronts

The brake master has nothing to do with bias. A larger master makes for a stiffer pedal is all. Doesn't change the bias

You can get two brake masters (like the Willwood competition units) linked by a balance bar, and adjust bias with that

What about tyres? Do you have the same tyres front and rear? Tyres makes a huge difference in braking

The way used by all auto manufactures is to fit less powerful rear brakes. OEMs that fit a Bias/Proportioning Valve do so to adjust for load (fully loaded/light loaded ... adjustable). The Nissan ones are automatic (load sensing)

Some of you may have read in my build thread that I am having some brake bias issues. I have the Maddat 7/8" master with R31 rears and S13 CA18 fronts with 250mm rotors. Installed a Wilwood proportioning valve which still did not reduce rears enough to stop them locking before the front (something like 50% reduction).
Going to try SR20 larger single pot with 280mm rotors to increase braking capacity at the front, before I go down this path, does anyone have any different ideas? Has anyone been through this before, or any suggestions on another brake master? I'm open to any suggestions. Cheers.