Oil pressure problem?

So I finally figured out my problem! when I put together this motor, I purchased the Mopar valve covers which came with the oil fill caps only. I installed a rubber grommet on one side with the pcv valve, but only used the fill cap on the other side. Well after changing my oil and driving around a little bit I wanted to make sure there were no leaks so I popped my hood and with my car off I heard a weird hissing, upon taking my oil fill cap off it made a woosh sound as though there was to much suction inside my motor. So I went to an autoparts store and replace the oil fill cap with a breather cap, and my car runs like a dream now! the oil pressure is up to 20 psi at idle, and now when I put it in park, it doesnt jump to 1400 RPM! Now I only hope that it only messed with my carb and not anything internally! -Kory