AC/Heat control inconsistency

[gw33gp]

A few weeks ago the air flow into my cab would switch to the defroster vent when I stepped on the accelerator pedal no matter which position the control knob was set on. I finally got around to diagnosing the problem and found the vacuum reservoir was not functioning properly. It did not have a leak but there is a check valve inside that is suppose to keep the vacuum at its normal level when the vacuum of the engine drops (as what happens during acceleration). This check valve stopped working properly and would bleed off the vacuum while accelerating causing the AC/heat ducting system to go to the default mode, which is defrost.

If you ever have a similar problem, that is a possible source of the problem.

 

[tazzryan]

i think the heater core is clogged & thermastat neads replacement.
anybody knows how too flush a heater core!!