Hi Rob,
 
Early SW20's had 7/8" bore master cylinders, whilst late had 15/16" bores. Non ABS cylinders have longer pushrods too afaik.
 
What you have described could be due to a few things, but i would be suspecting a booster or master cylinder issue. Can you please post a picture of your brake booster and master cylinder for better identification.
 

Hi Rob.
I have 1998 bone stock SW20 with ABS.
Master cylinder is 15/16 inch (cast on top under reservoir)    8B05   W25   (stamped on end).
Booster  834-02110     8205   (permanent metal sticker)    62  (painted on large sized)    Booster has NO check valve.
Check valve is in vacuum line in engine bay.
I have heard that a 1 inch master cylinder and the late model booster and check valve is a good combination with the bigger calipers.   Trav  should be your man.
Cheers.

Before I forget, I should've also mentioned that I do have ABS. 
 

Falcon
Hi Rob.
I have 1998 bone stock SW20 with ABS.
Master cylinder is 15/16 inch (cast on top under reservoir)    8B05   W25   (stamped on end).
Booster  834-02110     8205   (permanent metal sticker)    62  (painted on large sized)    Booster has NO check valve.
Check valve is in vacuum line in engine bay.
I have heard that a 1 inch master cylinder and the late model booster and check valve is a good combination with the bigger calipers. 

 
Thanks for the detailed info Doug!
 
It sounds like I'll need to have a closer look tonight and take more photos to post up. 
 
You said that on your car the master cylinder bore size is cast on top under the reservoir...should it be visible without removing the reservoir? I'm just wondering what to expect when I get home tonight and have a look. When I took the above photo earlier this year, I tried to find markings anywhere on the master cylinder but I could only find the "W8 OD27" on the front. Maybe I missed the bore size casting under the reservoir. 
 
I know that the 1 inch master cylinder and late model booster is meant to be the best combination, but I'm hoping to confirm exactly what I have now before I change anything. I may still have the original Gen 2 turbo brake booster and master cylinder, but I don't want to make that assumption because I need a known reference point for the pedal feel. 

I had a closer look tonight and my questions have now been answered. I just found the '7/8' marking hidden under the reservoir, so I must have the original 7/8 inch master cylinder, and therefore I probably have the original brake booster too. I never would've found that marking cast into the master cylinder if you didn't mention it Doug, so thank you!
 
I've attached some more photos just for the sake of it, but they aren't really necessary since I've now established which master cylinder I have.
 
This explains why I've found it difficult to be consistent with my heel-toe downshifts. Currently when I brake reasonably hard, the brake pedal sits lower than the throttle, so I have to move my entire leg (not just my ankle/foot) to get my heel up and over the throttle. I always thought it was just me, but hopefully this new discovery means that I have a valid excuse lol. With a firmer brake pedal, it should be at roughly the same height as the throttle which will be so much better!
 
Now I know what my next modification will be!

Today my new 1 inch master cylinder and 1995 SW20 slimline brake booster were installed. I also fitted a 10mm check valve in the vacuum line as required. The master cylinder that I ordered off ebay had a different reservoir with a different plug on the brake fluid level sensor, but we were able to do a direct swap of the reservoir from the original master cylinder so it was all good. 
The brake pedal is now much firmer and at a good height, exactly as I wanted. It feels perfect on the street so far, but I have to wait for the roads to dry up so I can try some more aggressive braking.

Falcon
Pleased for ya.  If your new vacuum check valve is plastic just be aware that they can be cracked quite easily.
Do NOT over tighten the hose clamps!
Rob if when you really start heavy braking it causes significant lockup at either front or back then you will need to pay attention to the proportioning valve.   Maybe a new late model one.   Wish you well with it.   I'm still keen for a ride.

I got a metal check valve so no worries there.
The ratio of the front to rear brake disc sizes is now the same as stock again, so hopefully the bias will be ok.
I owe a lot of people passenger rides at MTC! lol

With great power come great responsibility to supply passenger rides

Currently out of stock of RX8 rears, new shipment arriving soon.
 
Plenty of Supra kits in stock.