AC heater core blockoff valve leak rate?

So was it at least cool without the clamp down on heater hose? Did you have the contols in Max cool/max fan? (I think that water valve only restricts at those settings but i am not 100% sure).
Was coolant flowing thru the expansion valve? Was the compressor cycling or just always engaged?

Had you already converted to R134?
Did you change anything out when you evacuated the sys?(dryer)
compressor oil and seals?

What were the suction & discharge pressures when it was operating?
If the system was operating properly at about 2200 rpm you should have 15 - 18 psi on the suction side and 150 - 180 on the dicharge side (depending on ambient temp.)
If you have a guage set you should see about ambient + 30 degrees.
R-134's heat transfer rate above 100 degrees f. falls off quite a bit but I'm assuming that you were not seeing triple digit temps. this time of year.
If the heater hose valve is bypassing enough to alter the temp at the supply air grille by more than a couple of degrees then you have a problem which will be intensified as the ambient air temp. goes up.