brake booster.

Pain in the butt to swap. I had a 10mm socket disappear, then one of the four retaining nuts. Gotta lay under the pedals. Frickin bermuda triangle for parts and tools.....
Not to mention you gotta know the secret trick to get it out of the engine bay. Hint: the cutout along the right after moving the booster away from the firewall is the exact size of the plunger that connects to the pedal. Those tricky Japanese....