i thought the mono valve was something that has to do with the heater???? Anyway, there is a vacuum operated diaphram that most likely has a leak, and yes, unfortunately, it is in the back of the dash. You have to pull the entire dash(not fun, i know from experiance) to replace it. I would also not be suprised if you had moisture forming on the outside of your windshield with the a/c on due to this. To have this repair done, 900 sounds a little high but not un realistic.
96 integra SE....sold
99 a4 quattro....sold
2004 highlander limited....current.