Intex B Type plunger valves?

I'm sort of a newbie to soft side pools. On our second year, but our second and bigger pool. It's a Summer Escapes, but I cut the liner to add an Intex Sand Filter. After trying to vacuum, but pump cavitating, thus getting no where, I remembered that the Plunger valves came with plastic caps for the relief valves, so I put those on, an Voila! Now I can vacuum, but my pressure drops to 0-1 from backwashed pressure of 5. Is there an explanation for this? Is this OK? If not, is there something I can do to fix it? Also, is there any good reason why I should remove the plastic relief valve caps from the plungers when I'm not vacuuming? They are so small and so invisibly clear I'm afraid to lose them, so I would prefer to keep them on if I can.