To a guy who only lives about 30 miles from the REAL yellow brick road,
(and also has a yellow brick) what yellowbricktoad says sounds right to me.

I had that on my 164 a couple times, first time in about 1973, second
time a dozen years later. I can tell you that the brake fluid does NOT
help the booster either! The rusty old DOT 3 tends to take the paint off
and make the booster rust internally.
(another reason NOT to use DOT3, and I suspect that DOT4 is not too much better
in that respect!) I changed to DOT5 after the second occurrence and still
have the same master cylinder in the car 11 years later. Same fluid also,
still pretty clean.
--
George Downs, Bartlesville, Oklahoma, Central US