Originally Posted by rwenzing
poor brake assist after an engine start;
improves after a high vacuum decel:
improves after a medium vacuum cruise.
How about a leak in your brake booster? It has a fairly large volume that has to be "pulled down" by engine vacuum. In a healthy truck, you can feel that happen through the brake pedal right after the engine starts. If there is a leak in the booster diaphragm or a leak/restriction in its supply hose, it will take longer to build the vacuum necessary for power assist.
I just tried this my my truck and my moms 04 4.6 explorer.
Before i started the explorer, i put my foot on the brake pedal and fully depressed it. I then started it and the pedal felt like it depressed even more, even felt a little squishy as this happened.
Tried it in the ranger and the first time i started it with the brake pedal fully depressed, nothing happened - pedal didnt depress more or feel squishy. But the second time i tried it, the pedal did depress down a little but not like my moms explorer. The pedal did feel a bit squishy as it depressed but only depressed 1/2 as much as the explorer.
I guess i could still have a vacuum leak in the booster, just a minor or 'unpredictable' one?