The rear brakes on my Loyale are shot, I'm looking at replacing with disks from an '87 GL.  I've searched the topic, found a lot of discussion on proportioning valves, and complaints that the rear disks lock under low traction conditions.

 

I seem to recall that disk brakes move different (more?, less?, I forget..) amounts of brake fluid, and so I wonder, when getting the GL brakes, should I get the master cylinder too, and avoid the rear locking problem?

 

bk

The rotors are the same. There are about 3 different calipers, but all fit the same. You should not have to replace the MC. The proportioning valve is located ahead of the gas tank on the rear seat roll pan near the fuel lines. 

 

I wouldn't bother with the proportioning valve unless you have brake balance issues. But you may as well snag it from the donor car just in case.

Just did this swap on my 90 Loyale, with lift and 27.5" tires. Works great. Much better stopping power.