Toyota Repair: 4runner coolant leak, coolant leak, heater valve

Question
Ted, I have a 1988 4runner, 3.0 automatic transmission.  I have developed a strange coolant leak.  Depending on weather conditions it will leave a slight puddle under the vehicle, coming down the drivers side of the block.  I took it in to the shop and they pressure tested it and idicated a leaking heater valve.  They had pressure on it for 4 hours before it leaked.  I replaced the valve and it still leaks on ocassion.  

Any ideas where a leak could originate from?  It is concentrated on the drivers side block towards the back.  It only leaks if not driven for a little while and it sits for at least 24 hours and there is a significant temperature change.  I can see a coolant drip form at the bottom of the transmission bell housing and/or the oil drain pan bolt.

I can't figure it out and the shop couldn't either.

Thanks.

Answer
I haveen headgaskets leak at the rear of the cylinder heads, this engine has a water transfer pipe in the center between the two heads, it may be leaking and filling up the center until the water runs out on the rear, to inspect this pipe the intake manifold has to be removed, I would take it to another shop or if possible the toyota dealer and have it pressure tested again, it should not be that hard to find the leak.