I recently replaced all four ball joints on my 68 coupe with TRW products. Three of the ball joints accept the new grease while purging the old grease. However, one of the ball joints will not purge out the old grease. The rubber boot swells as I pump in new grease but no old grease comes out. Any ideas as to what is causing this problem and suggestions how I might correct this? Thanks for any help provided. Joe.