While troubleshooting the cause of hot air blowing from side vents while climbing a long hill with the accelerator pedal to the metal, I noticed that neither defroster vacuum element tubes could hold a vacuum, when the other tube has its normal connection to the switchover valve assembly. The defroster element has two vacuum tubes for a short and long stroke. However, when I disconnected and blocked the other tube, the first tube held vacuum properly both ways.
Is this element working properly?

Please Advice.