I have an 83 LE and I believe the Master Cylinder is either bad or needs bled. I can get ahold of known good Master cylinder from an 85 base with rear drums for free. Are these master cylinders interchangeable? I don't think people have to change the master cylinder when swapping rear ends? Though I am unsure, so i am quite curious.

 

The GS and GSL master cylinders have different part numbers. They are however more or less interchangeable per my understanding. I know people on here have run a GSL rear end with GS master cylinders without issue. My main concern would be the brake line routing and making sure that was identical before proceeding. Not sure if that was changed on 83 vs 85.

 

I had a Triumph Spitfire that I dropped a 13B in and it had drum brakes. When I did a custom CVJ axle I went ahead and did the disk brake conversion. The stock master cylinder didn't have enough pressure to stop so I had to get a RX7 master cylinder to fix the issue. I would check the bore sizes between the two before you get out there and discover you can't stop LOL

 

The disc brake master is slightly different and does NOT have a residual pressure valve on the rear circuit. The proportion valve is also a little different.
To use the drum master with the disc rear just remove the residual pressure valve. Its on the front of the master under the fitting the rear line goes into.