What shape are the secondary vacuum diaphrams in and is the linkage set right so the outers can open fully and get pulled closed when you back off?
When the diaphrams start to perish they go a a bit miss shaped instead of being nice and tight across the surface.
That can cause problems opening and closing.
The linkage on the drivers side should pull the secondaries closed though if they are tending to stick so usually a problem with the diaphrams only shows as an opening problem.
I have just had a curry washed down with plenty of beer so this will probably not make sense in the morning.