Two problems related to brake fluid leak have been reported for the 2007 Toyota 4Runner. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2007 Toyota 4Runner based on all problems reported for the 2007 4Runner.
Vehicle purchased used July 2011 w/ 45k miles. In may 2012 at 55k had brakes repaired, all 4 wheels new rotors pads and hardware. In 2014 noticed brake fluid leaking from caliper and discoloring wheels. Dealer service verified front calipers frozen (guide pins and pads rusted in place as well as caliper pistons). This appears to be a design flaw as the caliper does not incorporate any protective boot for the guide pins or any interface between the pad shoe and the caliper body. The caliper body and guide pin appear to be easily oxidized, a fact that Toyota attributes to "environmental conditions". Apparently Toyota designs for the mildest conditions rather than the harshest. Oddly, it is only the front caliper with this design: the rear caliper incorporates a more modern design with a boot for the guide bolt.
No brakes: brake pedal went all the way to the floor without vehicle stopping. Upon my personal inspection at time of incident, I found the brake fluid reservoir to be full and no brake fluid leaks,anywhere, were found. The Toyota service manager where I bought the vehicle stated that cause was a defective master brake cylinder assembly. I believe this is a manufacturing quality issue as I have not heard of a master brake cylinder assembly malfunctioning within a two year time frame.