jamesh

It took me 5 years to get to the root cause of my brake problems with my 2003 Impreza WRX so I'm posting this in hopes it will help someone experiencing the same thing.

Symptoms: Very poor braking performance after a cold start. The car would only glide to a stop even with the pedal at the floor. After the car warmed up normal braking felt OK but hard and emergency braking performance was terrible.

Troubleshooting:
I replaced the brake fluid, master cylinder, vacuum hose w/check valve, inspected calipers, etc, to no avail. Replacing the pads with non-ceramic helped cold braking a bit but not nearly enough. Over time it got worse and moving to high elevation made it worse still.

Early on I suspected the brake booster might be jacked up but it passed the recommended tests (pedal sinks after starting engine, held vacuum after turning off engine). The forums are full of complaints about poor cold braking with Subarus but the brake booster as root cause was always downplayed. They never go bad and when they do they fail completely, they said. Just spray some WD40 into the check valve; pump the brakes a few times and you'll be fine

Also there was the issue of the pedal going to the floor which normally indicates a MC failure or air/leak in the lines. It wasn't until I connected a vacuum gauge to the booster that I realized the pedal wasn't going to the floor. It was reaching a point at which the booster no longer provided any additional power assist. So what felt like the floor was actually the start of unassisted braking. This is why normal braking was OK but hard and emergency braking was poor.

Solution: Finally I replaced the booster and the difference was night and day. All the symptoms were gone! Cold starts (high idle + auto transmission) and higher elevation (lower vacuum) demanded more of the booster and just made the symptoms more obvious.

Bottom line: don't rule out a defective brake booster!