i have a 1998 jeep cherokee sport. 6cyl.4.0L.after having noise from the driver side wheel when applying bake.i changed the brake pads,rotor,caliper.also purge the line to make sure there is no air.still when apply brake sharp on 25 and faster mile i get a sound like FRUUUUUUU.from the driver side.does not do it if i apply bake slow..sound like a balloon or ball air coming out.i think it may be the master cylinder or the power booster..sometimes brake pedal is softer that other time,however,it does brake and pedal does not goes all the way down.any idea?