Quote Originally Posted by SLUF View Post
Looks like I have a good find on a disc brake rear axle assembly for our POS-7. Any one swapped out a drum rear to a disc rear on a first gen? Any trouble spots beyond R&R? I need to do a bit of research on the existing braking system to see what else needs to be changed (proportioning valve, hard/soft lines and such). Just wanting to see if there is some knowledge to be gained before I start on the project. Thanks!
You had drum brakes??? No wonder you had braking issues!

What year is the Disk Brake rear end?

This matters because there are differences between the rear axle housing and drive shaft flanges depending on year.

Differences.

84-85 models have bigger axles and bigger axle bearings
79-83 models have smaller axles and smaller axle bearings
79-82 models have smaller drive shaft flanges
83-85 models have larger drive shaft flanges

Interchanges

The third member will fit either the big or small bearing axle housing - the side gears in the third member are specific to the size of the axle.

The drive shaft flanges interchange on the pinion.

Mazdatrix list different part numbers for master cylinders for cars with rear drums vs cars with disks. Regarding the prop valve I would check the Mazdaspeed web site. You can see part numbers by reviewing manuals for 79-85 mazdas.