Before ordering / buying more parts, there is a test you can perform on the MC to see if the piston seals are bad. Cost ya about $20.
Interested?

 

Sure. Whatcha got?

 

Basically, you will be testing the MC only. Not the Prop Valve, not any lines, not the calipers. To isolate the MC from the rest of the system you must remove the lines on the side of the MC. But wait!

You will need two steel test brake lines. At NAPA for example, they sell pre-assembled brake lines about a foot long. Each end is already flared and have brass fittings.
On C3 MCs the two outlet ports do not match. Hence the reason you need two lines.
Off the top of my head, I do not remember the diameter or threads per inch of the front port or rear port. Seems like one was 1/2-20 and maybe 9/16-24? IDK
Maybe somebody will chime in here with those specs.

Take that information to your parts store and match up the two lines. Install those in their respective ports. Carefully bend the lines upwards. You can pinch off the ends or get more fittings to block off the ends.

Now stand on the brake pedal. It should not budge one iota. You are pressure testing the front & rear piston seals. If the pedal moves, there is your problem, look no further.

As a side note: The entire system will now have to be re-bled, you just introduced air.
Also, DO NOT use a bolt to block the MC ports. There is a casted flare inside that will be damaged, rendering the MC useless.
Keep the test lines for bench bleeding another day. They work much better than those cheap plastic tubes.

 

Makes sense. I made some shorts to plug the rear lines when I was redoing the rear end so I know exactly what you’re talking about. My new control arms just arrived so I’ll do the MC test while I’m doing the front rebuild. Thanks.

 

The brake line material of today is much more flexible than decades ago that's for sure.
Often one can bend & shape tubing w/o a bender tool and w/o kinking.
Unfortunately, my two test lines are of the stiff stuff.
Good luck, you will get it.