Providing this PSA due to the fact no solution found on this site or any other, which is troubling, considering the severity and no one*bothering to follow-up on the*more than dozen posts found throughout the Internet. [rant\]

PROBLEM:
When temps drop below 20 degrees F, second or third time brakes are applied after cold start, there's no power assist AND CANNOT STOP. After pumping several times, all returns to normal. In my fiancées case, she ran into a fence because, well, she's a woman and didn't think to invoke any of the numerous counter-measures at her disposal. I experienced the same problem on a different morning. What's important to note is someone else may not hit their brakes for the second or third time until rolling down a busy street.

DIAGNOSIS:
After many hours of research, here's what was happening: The vacuum check valve for power brake assist holds vacuum for first few times pedal is pressed when motor is off. On cold startup adequate vacuum is available within a few seconds, however in this case the valve would stick closed and prevent replacement of spent vacuum after second or third pedal depression.

SOLUTION:
Some say to spray WD-40 into the hose to absorb moisture, but I assure you this is not recommended on several levels. The inexpensive check valve is operating outside of design spec and should be replaced.

Depending on year and model of your car, replace check valve on vacuum hose between valve cover and brake booster housing. In the case of the 1999 Civic EX, valve is internal to the hose, so just relpace the hose assembly -- a mere five minutes of effort.

P/N & SOURCE: 46402-S01-A11 replaces 46402-S01-L11 $15
http://www.hondapartsnow.com/genuine...2-S01-A11.html