I would like to make an addition to this list. I recently changed both heater valves & hoses on my 940 & 740 turbos 1991s. The coolant return pipe on both leaked around the welded support bracket on the pipe. This is close to the turbo so whether heat or a road salt collection area I don't know. Both pipes were less than paper thin at this point but still had paint most others. Just the bit of movement caused by changing the hoses caused them to leak. The pipe is best seen from under the left side near the oil filter.

Steve is giving good advice, don't be cheap change the heater valves etc. Mine snapped in half when I removed the hose clamps. A 5/8" hole dumps a lot of coolant in a hurry at 20PSI.