Replacing a Vacuum Check valve in the HVAC system
Even after repairing the main vacuum line in my HVAC controls I still had the problem that under prolonged acceleration, the vents would loose vacuum and switch to the Defrost setting. This problem is often caused when someone deletes the vacuum bottle behind the front bumper. However, my bottle is still in place behind the stock bumper.
I studied the vacuum diagram and saw there is a check valve where the source connects to the intake manifold. I tested mine by blowing through it. I could easily blow both ways so the check valve is bad.
I found a set of VW check valves on Amazon so I got those instead of the exact replacement for the Jeep.
The replacement valve is slightly larger but fits fine in place of the original one. Now the accumulator bottle can do its job while accelerating.