I have a 2002 Mazda protege. I recently bought from a friend. It has what I believe are Mazda speed protege front calipers and rotors and I think Mazda 6 rear calipers and rotors. The car is heavily modified except for the motor. All the suspension and sway bars and control arms. I’m locking up the rears much quicker that the fronts driving the twisties. This makes the car hard to control under braking and very hard to control under emergency stopping conditions (the rear wants to step out quick). Im curious if anyone else has experienced this and the remedy. Obviously it’s because the rear brakes are big and powerful so I’m thinking adding a brake proportioning valve is the answer but wanted to ask here first. The only other alternative option would be downsizing the rear brakes.

 

Welcome to the forum

 

Your brake bias is not working correctly because of the size difference in the rear brake parts over stock . The best solution is to return to OEM complete rear brakes.
The other solution if you want to stay with your mixed front and rear brake parts is to install a manually adjustable brake bias vale. But if you're not in any way experienced in measuring this equipment then it's not for an average DIY and most service shop will not install them either. This is an aftermarket performance only part.