Master cylinder not working after hard stop

Yes, but not from the pressure. Crud and corrosion build up in the lower halves of the bores in the master cylinder where the pistons do not normally travel. When you are surprised by a leak and run the brake pedal to the floor, the rubber lip seals run over that crud and that can rip them. Often this results in a slowly-sinking brake pedal that does not show up until two or three days later, but it can affect both pistons right away, as likely happened here. The same problem can occur when do-it-yourselfers or inexperienced mechanics pedal-bleed the system with a helper. When doing that, it is important to never run the brake pedal more than half way to the floor.

Typically you can only buy rebuilt units. New ones are very much more expensive. You can buy rebuild kits too, but I do not recommend them unless that is all that is available. By the mid 1980's, all master cylinders were made of aluminum which corrodes very easily. To prevent that, the bore in the master cylinder is sealed with an anodized coating. It is critical that the coating not be damaged when cleaning it. That means a rag, Brake Clean chemical, and at most, a soft bristle brush. Absolutely no wire brushes or metal picks or scrapers.

Don't forget to bench-bleed the new master cylinder.

Here's a trick that can save you a real lot of time. I forgot to post this earlier. This may not work as well on Fords that have four steel lines leaving the master cylinder, but it works well on all other car brands. Slightly loosen the two steel line nuts on the master cylinder. Unbolt it from the booster, pull it forward, then use it as a handle to bend the steel lines upward a little. That will prevent the fluid from running out of the lines. Remove the lines, connect them to the new master cylinder, use it to bend the lines back down and install the new master cylinder to the booster. Snug one line nut and leave the other one loose. Have a helper slowly push the brake pedal half way to the floor. It should take about ten seconds for him to get there. When he hollers he's ready, or you no longer see air bubbles coming out, snug that line, then have the helper release the pedal quickly. Loosen the nut and do that a second time or until no more bubbles come out. Do that for the other line next.

By pushing the pedal slowly, fluid gets pushed down into the line and air has a chance to float back up. By releasing the pedal quickly, the fluid rushing back will wash any remaining air bubbles up into the reservoir. You'll never have to bleed at the wheels this way, or risk breaking off a bleeder screw.

Be extremely careful to not get any hint of petroleum product like engine oil, transmission fluid, power steering fluid, or penetrating oil anywhere near parts that contact brake fluid. That will destroy all the rubber parts in the hydraulic system.

Actually, new ones are put together assembly-line style so they have a low failure rate. Rebuilt units are hand inspected, then hand-assembled individually. There's more chance of operator error, but each one gets special attention to detail. It's a horse-a-piece which is better. The rebuilt unit is likely to have been assembled with "Brake Assembly Lube" which is just a thick brake fluid. They would do that because they know it could sit on a shelf a long time before it gets sold and installed. That lube keeps the seals from drying out. I suspect there's a chance a new one doesn't get that treatment because it is expected to go on a car real soon.

I would have no problem going with the rebuilt master cylinder, especially since it saves a few bucks. I've never heard anyone say their rebuilt master cylinder failed.

Yah, I have lots of people fooled about my apparent wisdom. Actually, it was one of my specialty areas, then it was one of four areas I taught for nine years. You were just the lucky recipient of my great and wondrous knowledge.

Please keep us up to date on your progress.