I have an old Toro proline 52 inch commercial walk behind 18hp kohler and I don't know how to put the transmission belt on right.

most of these switches areblocated under the mower along the brake pedal shaft

did the belt break or fall off?are the push control rods pulled out to push it ?

sounds like a safety switch is bad check the one on the seat first

try starting fluid that will see if its a fuel problem then check for spark

if it has a electric clutch check for power to the clutch if no check the safety switches

the should be a plastic cover under the engine

May I get the make and model # to help you more

I suspect the drive belt, not the Hydro. Especially if the drive belt was never replaced. The belts are known to stretch and wear out over time. It's a cheap part.

try a new spark plug

Wait until the mower cools off

1. Remove the drive belt cover from the top of the mower
2. Tip the mower up, being careful to not go too far or the oil and fuels may spill out
3. Remove the mower blade - as I recall the bolt is 5/8" (could be 9/16") You will have to hold the blade from turning (if you have impact tools, this will be much easier)
4. Remove the blade
5. Pull the pulley out or off and remove the belt - be careful to not lose the crankshaft "woodruff" key - it's the little piece of half/moon shaped metal that keeps the pulley from spinning on the crankshaft.
6. Reverse the steps to reinstall

You MUST make sure the blade is tight when you reinstall it. Also, the blade usually fits over a couple of pins - make sure they are lined up properly and the blade is installed right-side up.