My first post. I have a YS4500 22hp B&S riding mower about 8 or 9 years old. The hydro-stat transmission has developed a problem with engaging properly. You can move the handle slowly upward and nothing happens until, BAM, it suddenly leaps forward. My wife mowed with it last time and told me about it. It seems to have happened quickly and not over a long period of time. I have checked the fluids in the transaxle and pump and the levels are OK. The friction ring and caliper seem to be OK. Any ideas as to what it might be? Thank you for any replies.