I am just repeating here what I have read elsewhere on Toymods, so search for the original page for more info.
Since the car came stock with rear drums there is a proportioning valve which does something funny (yes that is the technical term) to the F-R bias. When you change to disks you have to rip the guts out of it otherwise the rears lock up early.
The proportioning valve itself is the block that the two lines out of the master cylinder go into before heading off to each corner of the car.
So just check it out before you spend big dollars on some aftermarket bit.