Why do my drive pedals on my 855 not return to position when released?

I have a jd 855 hydrostatic drive. I just got it running and it is running a bit rough. The problem I have is tht the forward nd reverse pedals do not seem to have any pressure as they do not return to their positions when my foot is removed. When depressed, the tractor will move but really bogs the engine down, even causing it to stall out. the bucket and pto and draft all work. The hydraulic level shows it is filled to proper level. The tractor is a 1998 model with 440 hours.