I'm fairly certain on the cause of this one. Finally got the rest of the brakes replaced and bled. Fired it up and applied the brakes...PSHHHHHHHHHH. Pedal was low but firm. I think all signs point to failed booster, right? Between the noise on application, and the fact I've already replaced an MC due to leaking fluid out the back, I think it's more or less a done deal. I am curious if there's any tips or tricks as to preventing recurrence.

 

No tips on preventing a booster malfunction, when the unit is done its done. When the brake pedal gets stiff its stiff.

 

I had what I would consider a minor failure with the one from my 1970 442. I still had power brakes, but the engine was running like garbage because of the vacuum leak. I could hear the leak inside the car and when I opened the hood...wow. I pulled the booster hose off and capped it with the thumb and the engine smoothed right out.
It was odd though, that I still had some power assist when the hose was connected.

 

Oddly enough while it's hissing, it still seems to be functioning. I removed the hose and plugged it while it was running and there was a massive difference in pedal effort.

Oh well, I'm guessing it's on its way out anyway. Might as well get a new one.

 

That sounds exactly like how mine was acting, but the engine was idling badly as well.