97 Suburban coolant leak

My 97 Suburban 2500 5.7L leaks coolant (sometimes) on the passenger side right behind the air cleaner. I took it into the shop twice to get it fixed, the first time they replaced my water pump but the leak continued. The second time they claimed it pressure tested fine, no leaks they could find. It seems to happen more on warmer days and tonight I noticed it leaking quite a bit when I got home. It seems to be leaking from sort of vacuum valve(?) connected to the heater hoses. I'm not sure what this is but it seemed pretty clear tonight that it was the problem. Exactly what is it and is it pretty easy to replace?

Thanks!

Do you have the optional rear heater installed in your burb? That valve is probably a bypass valve that directs coolant to or away from the rear heater core. In any case, what your describing sounds like a vacuum operated coolant bypass valve. Its definitely available from the dealer for $$, try contacting a company over the internet that specializes in automotive Heating and Cooling parts.
Here are a few to try: www.ackits.com, www.aircondition.com, www.acsource.com, keep-it-cool.com, www.acpartsonline.com, www.aircoparts.com, www.acpartsnow.com
At least one of these may be able to get one to you cheaper and possibly of better quality than the dealer. Hope this helps -Chris