One problem related to brake master cylinder has been reported for the 2004 Jeep Wrangler. The most recently reported issues are listed below.
My daughter experienced brake failure in her 2004 Jeep Wrangler causing her to coast through a stop sign and hit an oncoming car. Upon inspection of her Jeep, there was a clip missing that secured the connection of the brake pedal assembly to the actuator rod that goes into the brake booster/master cylinder. Without the clip, the two parts separated causing complete brake failure. This car has no history of brake repair in that area which indicates a faulty clip or a clip was never installed from the factory.