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?

 

Sounds like a problem with the electronic hydraulic control unit (HCU)

Is there any ABS light warning illumination when the problem happens?

 

Thanks, Found the HCU, researching how to test it, if possible.
Yesterday AM, the last time it happened the brake light came on and stayed on.

Addl information:
Basic booster test (at the pedal) does not indicate a problem.

Check engine is showing the "lean" code which led me to think this could all be vacum related.
- Main vacum checked out ok: consistently at 23 at all RPM.
- vacum at pcv only builds to around 7, not sure if that could be the problem.
- The two hoses that go into the thing on the egr side (air sensor?, PBTGF30) show no vacum at all. Not sure if that's right or wrong.