Bad Brake Proportioning Valve???

[zipgun]

Hi all,

My 90 Burb has been front braking really weird. I have never had a prop valve die on me so here goes. When the brakes are cool and during the first stops it does fine but after they are used awhile the front end seems to grab in each direction alternating grabs. I do not believe the rotors are warped (truck has 123,000 miles on it) and from what I can tell the caliper pistons seem to be operating alright. Anyone ever deal with this or experience the same symptoms ?

Thanks all,

Steve