Hello. I bought a project off a friend and it's a 1996 escort si. He had a rs2000 rear beam fitted to it and I've tried to put it through it's MOT today and it failed on rear brake efficiency. The tester said it was the master cylinder at fault and it wasn't designed for rear discs?

Can I just change the reducers that are attached to the master cylinder?

 

i put rear discs on mine left the same master cylinder on and worked rite away. check the hoses