Hi All,
Currently I have stock 91T brakes but with next size MC - 15/16” I think. Overall the brakes are underwhelming. What I am thinking of trying, is simply to replace the front discs and callipers with the later 93+ version, which according to my arithmetic will produce about 15% more power in the front wheels.
I expect, but could be wrong, that the proportioning valve can remain unchanged. Has anyone tried this and did you get wheels locking prematurely?  
Also I would try the slimline booster, which apparently increases power, but that’s unrelated.
Cheers,

What do you mean by 'underwhelming', and what are you hoping to achieve?
 
From my experience with the early small brakes, by fitting a 15/16" M/C with your old booster you would have reduced pedal travel, but need more pedal force compared to with the OEM 7/8" M/C to achieve the same braking.
 
Changing to a later booster, especially a post 11'93 one would mean much less pedal pressure needed.
So for a better pedal feel, changing the booster might be a good first step.
 
 

Thanks Redd (again),
‘underwhelming’ as in stopping power. Toyota admitted as much with their revision.
Yes, the pedal travel is less and more leg power is required, so your suggestion to fit the later booster before deciding on the discs, makes perfect sense. I will do that.
 Is 2^nd hand ok for those things ? and is this the one where Toyota moved a checkvalve down closer to the engine ?
Cheers,

I still have the early small brakes, and at one stage I had considered going to the larger brakes because like many people, I didn't like the OEM pedal feel (long travel - spongy), but I soon rejected the idea.
 
Why?
1. The main reason is that I have done a few track days, and have never noticed brake fade even running stock Toyota pads. Of course my car is an NA, and a high powered turbo on the track could be a different story.
 
2.  Apart from the cost, the larger brakes would also add approx 4 kg to the un-sprung weight, (aghast!!) so why would I want that?
 
3.  For some time now I have run a 1" M/C along with a '97 booster (vacuum valve is in the engine bay off the intake manifold) that totally fixed the pedal feel. I bought my booster S/H from Asia with no probs.
I had first tried a 15/16" M/C with the '97 booster, but found it very easy to lock the wheels in a high speed stop on smooth dry bitumen even with the ABS turned off. That was a bit scary, so I changed to the 1" M/C so that I needed less pedal pressure.  That gave a great pedal feel with very little travel that was exactly what I was after.
 
 
 
 
 
 

Underwhelming could be because of many factors. Braking has many factors that contribute to the feel and result, such as when did you last do a full brake fluid replacement, pad type, tyre compound, bushings, springs etc before you get to main braking components like booster, portioning valve, caliper size etc.

Service your vehicle components that impact braking. Get along to a club or track day where you may have the opportunity to be in a later model Mr2 or brake upgraded car - that’s a short cut to seeing what combination meets your needs.

Often quality pads (in the heat range etc of your driving application) serviced items that impact braking, will provide a good result vs cost. If you are racing your Mr2 then that’s a different conversation due to dynamics of heat l, sprung weight, and speed ranges.

Reddtarga
I had first tried a 15/16" M/C with the '97 booster, but found it very easy to lock the wheels in a high speed stop on smooth dry bitumen even with the ABS turned off. That was a bit scary, so I changed to the 1" M/C so that I needed less pedal pressure.  That gave a great pedal feel with very little travel that was exactly what I was after.

Redd,
 Not sure how to proceed now..... Did the 97 SW20 have a 1" MC?
 when your 15/16" MC was causing lockups, were they front or backs locking up?
 I'm confused, when brakes are too powerful...isn't just a matter of using less foot pressure ? or is that easier said than done?
 I'm trying to work out how the check valve works in turbo. I guess it must be able to hold vacuum for a long time.
Unlike NA which goes to vacuum immed when you brake.

The 1997 is bound to have had a 15/16 inch master cylinder as that was the largest ever used from factory.
Lot of people go straight up to a 1 inch for using larger calipers or even the 93+ calipers.
I'm also a bit confused as I would have thought it possible to just modulate the pedal pressure to prevent lockup ?
Yes the vacuum being held by the check valve is able to remain for many hours after engine shut down.
I imagine the inlet manifold would drop to vacuum almost instantaneously on closing the throttle same as na.
I have two different instruments monitoring inlet manifold pressure and they both show an instant and dramatic drop from lots of pressure to lots of vacuum.
I have had plenty of difficulty trying to work out brake problems on my car so I'm particularly interested in any discussions that this may generate on here.
Does your car have ABS fitted from factory ?
Will PM you.

Your inbox is full so it won't accept my PM.

TwoDogs: "I'm also a bit confused as I would have thought it possible to just modulate the pedal pressure to prevent lockup"  
 
Well yes it would be, but modulation preferences vary with different people, and there are other variables as well.
I use stock Toyota pads that have plenty of bite, even when cold. I imagine 'performance' pads would probably be different. Also in my case keep in mind that I still have the early small brakes and not the later ones. 
 
With the 15/16" M/C, once I fitted the later model booster it made a huge difference to the pedal feel using the small brakes. I would even use the word 'deadly' to describe it compared to with my OEM '90 model booster.
I would have had to disconnect the ABS if I had wanted to keep using it, but being a road car I wanted my ABS operational. 
I happened to have a new 1" M/C sitting around from an earlier project so I didn't hesitate to try it.
Anyway, even forgetting about the ABS it turned out that the 1" M/C gave me the perfect balance between brake modulation, pedal pressure, and pedal travel. The ABS also appears to work normally with it.
 
 
 
 
 
 

Falcon
 
I have had plenty of difficulty trying to work out brake problems on my car so I'm particularly interested in any discussions that this may generate on here.
Does your car have ABS fitted from factory ?
Will PM you.

I'm sorry if this is getting away from the original subject.
Doug, I don't know if this may help your brake problems, but if you have heaps of spare time here is something that may interest you for your track car.
Front brake airscoops fabricated from 1 mm aluminum, and fiberglass that direct the air right into the center of the rotors.
With these fitted I have found the rotors actually run cooler under heavy braking!!
They have lasted a couple of years on my car without getting destroyed, and they don't foul the caster rods.
Ha ha, yes, I know. I just have too much spare time on my hands, but it's fun making things.