I'm replacing the master cylinder, hoses, and wheel cylinders on my '67. Not sure exactly what went wrong, but the master cylinder was leaking and brakes were soft for a long time, then all four wheels just locked up one day.

When I pulled the master cylinder, there was a yellowish gunky substance (about the consistency and texture of industrial strength hand cleaner) built up on the brake booster. Anyone have any ideas what this may be? I'll probably go ahead and replace the booster with the rest just to be safe, but I would still like to identify the mystery substance.

 

It's a combination of old age, and moisture. It's also possible that the master cylinder could havebeen leaking fluid on the booster side, which would be one explanation foryour soft pedal. Old style brake fluid can absorb moisture, and a car that sits is more likely to have it happen, especially as seals dry and shrink.It's also likely that youhad some rust in the system thatcould havedislodged or accumulated and causedyour master cylinder to lock up.
Fluid doesn't travel through thepower booster, so what you had wouldn't have had any ill effect on it. It's up to you if you want to change the booster, but it would be just for the sake of having a new one. If the old one is ratty and questionable, then I suppose now would be a good time.

 

Found the same gunky junk on mypowerbooster also. Mybrakes were also soft but nothing locked up.Theres a rubber"o" ring on the master cylinder's plunger assembly that will rot and allow brake fluid to reach power booster. Mix that in with dust, moisture, rust, etc.....ya got yourindustial strength mustard.
After a good cleaning, I keptthe originalpower booster (Delco Morane), but replaced everything else for safety.

 

Thanks for the input. I found someone locally who will rebuild the original booster for a decent price, so that looks like the best option.