Have not done the brake booster test yet. It's on my things-to-do list.
This is one of those problems that, when it shows up, the conditions are not very conducive to troubleshooting (20 deg in the morning). When the weather is more friendly, like in the spring and fall, the problem is not there.
hrmmm do you have a long hill by your house thsts straight and safe to try a test on?
With the car off pump the barkes 10-15 times. With your foot on the brake pedal start the car, the pedal should move to the floor. If it doesnt you have a bad booster. Do this test first thing in the morning when the car usually has no brakes.
After doing the booster tests (test takes 5 minutes so no excuse for NOT doing it). Park the car in neutral using the hand brake. Pump the brake 10-15 times. With the engine off, but the ignition switch in the unlock postion release the hand brake. once the car starts rolling a bit, press the brake pedal. does the car feel the same? You can start the car in neutral rolling then get brakes back. Remeber when you loose pwr steering and pwr brakes, you can still control the car, but it takes a lot more force to do it.
My gut feeling is that the car is not loosing brakes, but loosing the booster vaccum. Once the booster builds up vaccum its fine. I bet a pinhole leak is in the booster diaphram. The car WILL stop, but you may need alot of force or two feet to do it when it feels like you have no brakes.