If the swishing noise is coming from say the dash area infront of the driver you could have a failing brake booster (round componet that the master cylinder connects to) Also check fo leaking vaccuum liines.
Are you having to add brake fluid? The master cylinder amy be leaking fluid into the brake booster. Check for loss of fluid, booster being damp with fluid under the master cylinder and if you are mechanically inclinded and comfortable doing it, remove the master cylinder from the booster and look for fluid at the seal where the rod (piston) goes into the back of the master cylinder. This can be done without removing any brake lines.
There is a recall on 99 Explorers for a possible faulty brake line connection (see below) doesn't sound like it would cause your symptoms, but check with the ford dealer to see if your vehicle is included in the recall and if so has the recall been completed;
FORD MOTOR COMPANY
Campaign Number, Case: 99V123000, 99S16
1999 FORD EXPLORER, 1999 MERCURY MOUNTAINEER
No. Vehicles Recalled: 31362
VEHICLE DESCRIPTION: SPORT UTILITY VEHICLES. THE RIGHT FRONT BRAKE LINE TO
THE HYDRAULIC CONTROL UNIT (HCU) CONNECTION COULD SEPARATE CAUSING
THE LINE TO LEAK WHEN THE BRAKE PEDAL IS APPLIED.
THIS COULD RESULT IN A LOSS OF BRAKE PRESSURE TO THE FRONT WHEELS,
INCREASING THE STOPPING DISTANCE.
DEALERS WILL REPLACE THE FLARE NUT ON THE BRAKE LINE.
Let me know what you find or if you have any more questions.
Tuesday, August 4th, 2009 AT 12:54 PM