Damian,

There are 4 types of WUP. The vacuum line on top of the WUC (also known as fuel pressure regulator) is used for vacuum-controlled control-pressure to provide full-load enrichment and altitude compensation. It does not affect idle revs, but check that the contour hose on the aux valve in your car does not have vacuum connector inlet. There was a cut-out production date to eliminate this vacuum-control-pressure requirement (1979) and the WUC top vacuum line simply vents to atmosphere in some models.

Your symptoms seems more to do with the vacuum-switchover valve (active when air conditioning is on) to rise revs. Other posibilities are the fuel accumulator or the check-valve on the fuel pump. A series of tests would be required to ascertain de drop on revs when warm and sudden rise when load is reduced (gear shift in park).

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