I'm looking for advice. As far as I know, I'm running a stock MC and front and rear calipers and the front brakes lock quite easily under heavy braking.
It has always done it, with different pads, different sized wheels (16 and 17 inch), different tyres, even different sized rotors.
I know that the bias is set by the MC though I'm wondering why it locks with a stock setup.
I have bought an inline bias valve for the front circuit though I'm not 100% comfortable with restricting pressure to the front brakes.
Without buying a pedal box with separate master cylinders for the front and rear, what can I do to fix it?

Simple answer is not much. How is your suspension in terms of spring rate?
 
Are the brakes early or the late type?

Better tyre's could help?

I put my money on spring rate and/or damper settings...

it is just an mr2 thing. they do indeed lock up easily when you just mash the brake pedal hard. stems from the lighter front end unfortunately (or fortunately), you need to get the weight shifting to the front end before you you can hard brake.
 
  but if your looking for advise on how to solve it. i guess you you try looking into option like these but not limited to: 
 
1. didn't SW20's come with an ABS brake option. perhaps get one of these systems and install it.
2. add as much weight to the front end of the car as you can.
3. do you brake bias valve adjuster that you have.
4. as others have said dick around with springs rates.
5. learn to brake better. as said you can't just mash the pedal you need to apply it in stages a bit first to transfer the weight to the front wheels, then you can brake hard.
 
  If you don't want to try improving your skill the first 4 options will be the best for you. thats my experience coming from an AW11 anyway. Done some driver training with Rick Bates and number 5 was his advise. so go complain to him if you don't like it.

I skimmed over some chatter about this kind of thing on MR2OC once and I'm pretty sure there was some talk of giving more bias to the rear of the car due to it still having a large percentage of weight over the rear even with heavy braking. This means you should be able to get more braking tractive force (Correct term?).
 
This was mainly in relation to adding the SW20T rear calipers and 20mm rotor to the rear of the AW11 and then bumping the bias to suit. I assume if the information is right, that you could experience some better braking with less lock ups, but it might make the car pretty hairy to drive if your doing any moderate breaking in corners (The kink @ Wakefield).

kameleon

Simple answer is not much. How is your suspension in terms of spring rate?

Are the brakes early or the late type?

I haven't had the springs checked though they are pretty stiff. It could probably do with softer ones.
I'm planning on getting them checked and new springs fitted sometime next year.
Calipers on the rear are 22V. I'm not sure about the fronts. I'm running the big rotors front and rear. Supra turbo front and RX8 sport rear though the brakes locked before I changed them as well.

@MRTurbo I think you may be right - I need more weight transfer to the front.
 
@Wiso I have thought about ABS though it would just mask the problem. It shouldn't lock so easily. I may try the bias valve and also put some softer springs in it to help with the weight transfer.
I need to take weight out of the car - not add more!
I can drive mate - it's definitely the car.
 
@Knightrous yes the bias needs to be more to the rear with the current springs. I'll give the bias valve a try