Just finished replacing the master cylinder for my FC S5 TII with the Mazda 929 MC . Works great! Almost too great.
Rears lock up really fast. I still have ABS unit installed
Bled the system really well.
Pedal is good.
old MC had trashed seals and pedal would slowly go to floor.

I am wondering if the proportioning valve (PBV) could be the issue?
Had a Greddy FMIC installed many years ago and they moved the PBV off the mounting bolt.
I checked if they swapped the front and rear lines to the ABS and they look good.

Anyone use a manual Wilwood proportioning valve or other type?
Something like this?
Everything you need to install an adjustable brake proportioning valve on your Miata! – flyinmiata
https://flyinmiata.com/support/instr...ning_valve.pdf
Cost $65 for above
Stock new is $211
Used PBV unknown. but would it be worth it?
Repair/clean PBV - can it be done? since the MC seals failed could the PBV fail too ? does it have seals?

 

i don't know, but i've got some links

https://www.rx7club.com/1st-generati...valve-1159310/
https://www.rx7club.com/race-car-tec...-valve-945620/
https://www.rx7club.com/race-car-tec...-valve-931953/

 

Ah I only searched the 2nd Gen forums..
Thanks

Reading those links seems there is a good balance of going this route.
The Miata PDF install guide is great ins terms of guidance with install, bleeding and testing the balance.
It is also reversable..