I can't visualize where the leak is, when you say it's behind the exhaust. Can't be the water pump if your leaking slows down only after the reserve container has emptied, it has to be higher on the block. A common place where leaks can occur is the little rubber hose that comes off the thermostat housing and goes over the exhaust shield in front of the block. It's subjected to a lot of heat and needs frequent replacements.

2gnt: '99 RS-T, killed by a toyota, pending rebuild...
Daily: Volt
Daily #2: '99 EVG ebike- STOLEN by PEDRO