brakes - occasional rockhard with back-tapping
97 F150, 4.8, 217K miles
About once a month, at most, when I apply brakes, I find them rockhard. As I press harder, I feel something tapping back. After 10-15 (anxiety filled) seconds they start working normally again. So, far has only occurred within 2 minutes of start-up, and usually the first time I hit the brakes.
It's one of those occasional problems I really would rather not ignore.
Is this likely vacum related? Suggestions?