Toyota Repair: slow water leak, hydrocarbon emissions, head gasket

Question
85 Toyota 4-runner manual trans, 4 cylinder. 300,000 miles

After an hour or so of driving the water level goes down and engine overheats.

Is this a main gasket or engine leak, or could it be the water pump? I replaced the water pump several years ago with a rebuilt.

Answer
The first thing is to inspect if there is a visible external leak, you may have to have the cooling system pressurized, if there is no external leak there is a good chance that it has a leaking head gasket, you can have it checked for this at most repair shops, they can do a chemical test to look for hydrocarbon emissions in the cooling system