Brush hogging for two hours today and tractor dies. Put 3 gallons of fuel in and bled 1st line until it started. As I headed to shop I noticed a loss of power. Tractor would not run 10' brush hog and drive at normal speed, really bogged down. I disengaged pto and revved engine while stationary to 2600. Idled down and pulled into shop and killed it. Got out and noticed oil colored diesel spot on floor directly under blow by tube. Spot was a good 10" in diameter and really thin liquid. What should I check first? Thanks.