I have a Trek 820. Just had it adjusted at bike shop. He's almost 2 hours away so I need to fix this myself.
Went for a ride today. When I shifted into high gear (large gear on front sprocket) I felt the shift lever go a little spongy at the end of it's travel, but I don't think I pressed on it overly hard. If I kept pressure on the shift lever, the chain would stay on the large sprocket. When I released pressure on the shift lever, the chain fell back on to the middle sprocket. I don't recall now if the indicator stayed on 3 or went back to 2.
Anyway, please help. I want to ride my bike.