If master cylinder seal fails, what happens?
 

If the seal tears, do you lose your brakes entirely and drive into a wall?
Just wondering about the shelf life of this part.

Let's just say compromised. Your pedal will be soft but you can pump it up quickly to where it feels better.

There may be levels of being compromised?

However, you have a dual braking system. One side can go ti*s up, in the ditch, and the other side (seal) will stop you.

I am running my 1981 seals.

+1 w/ Bob there.

Mine didn't go catastrophic, but I suspected an issue, investigated and addressed it a couple years ago.

I found a small leak, and I never noticed it before I looked under the frunk carpet and looked under the dash below the steering column (or, beneath the master brake cylinder but in the cabin - there is a little plastic plug, and the bf dripped extremely slowly).

I found it by accident when I was refreshing my pedal cluster. My brakes seemed ok, but my clutch was a bit tough btw (slave cyl) and I thought I might be leaking fluid albeit very slowly. But I never found any on the ground. Instead, the black carpet took some, and the rest dripped to the smuggler box and into infinity. And we aren't talking a lot.

I replaced it, the master clutch and clutch slave cylinders, some old lines and even the reservoir as well. Brakes are fine and clutch pedal action is lighter btw.

I can't answer your shelf life question, but you can use me as a data point.

They fail slowly in my experience. But we once had a car in where someone had added power steering fluid or oil by mistake. Owner reported it failed quickly. Ended up replacing all rubber based components.