I just replaced the M/C on my '80 Malibu wagon. The instruction that came w/it reads as follows:
New and Improved Master Cylinder Bench Bleeding Procedure
- Tighten bleeder plugs into ports.
- Support M/C in a level position.
- Fill half-way w/new fluid.
- Using blunt tool, (brake pedal if mounted to vehicle) SLOWLY press in and release the M/C piston using short strokes ¾” to 1”. NEVER STROKE PISTON MORE THAN 1”.
- Repeat this step until resistance to piston movement is firm and less than 1/8”.
I did it this way and had no problems except getting the plastic threaded plug on the larger of the two ports to seal.
The plugs are "stepped", having both size threads. This was OK on the smaller port, but the larger port allowed the plastic plug to bottom out against the hex head of the plug. The threads were short, so I wasn't too confident it would seal very good against the pressure. There was no leakage of fluid, but it bothered me just the same.
Most GM master cylinders use 9/16-18 threads for the front ports and 1/2-20 threads for the rear ports. You can make bolts work for this, just be careful
to not damage the inverted flare seat in the ports. Wrap the threads real well w/teflon tape- this will tighten the fit of the bolt threads and help to seal the ports. Or use flared tubing and fittings w/the tubing soldered, brazed or welded shut.
What remains to be seen is, will this work on any/all other M/C’s.