I am able to start my mower, but when I put it into gear, the brake/clutch pedal comes all the way back towards me, and the mower will not move. The big spring is still attached, but obviously something is wrong. This happened while mowing over high brush. This mower is at least 15 years old.

Many Sears mowers make it to 15 but they do need maintenance from time to time. Lets start by checking the belts. If you tell me your model I can get you a drawing of the belts and there sgould be a decal on the deck. Or we can just talk our way through this. Get someone to help you. The someone puts the mower in gear (engine is OFF) and releases the clutch/brake while you watch the pulleys and tensioner move. You can see how the tensioner slowly tightens the belt that run to the transmission under the seat. Does that belt get fully taught?

I have an old craftsman mower also . I don't know if it's a safegard or not but have you checked the lever on the back of the mower . Identify while standing behind mower . On panel , see a push pull lever pulled out for pushing , pushed & locked in for mowing.

Thanks. It turns out that the plastic tensioner must have taken a rock hit as part of it was broken off. I couldn't see it until the mower was put up pn a jack. Replaced with a new tensioner and all is well.