Now I know that you are supposed to change the master cylinder and proportioning valves when you do a rear disc conversion. First thing does anyone know the part numbers for these two?

Also my main question is what will happen if I don't switch the proportioning valve and master cylinder? Will the brakes be really weak?

 

The only difference I believe, is that the drum brake proportioning valve uses or applies more % of pressure to the rear than the disc brake one would. This could be fixed by adding an adjustable proportioning valve in line. I don't know about the master cylinder though.

Hope this helps

 

I just swaped to the smaller rear disk off a 83 fireturd on my monte and have the drum proprotioning valve and mastercylinder still in place. You will have NO rear brake pressure. The darn thing left 2 big ole black marks outta my garage cause the choke was on and made the car power brake itself without the rear brakes. You'll need to get a disk proportioning valve or an aftermarket adjustable one. From what i have read on this board you don't have to swap master cylinders, in fact GM quit using 2 dfferent ones in the later 80's and just used the drum mc. Hope this helps.
Ohh yeah if you wanna know how i got the disk to work on my 9" let me know and i'll post a new thread.