Pontiac Sunfire clutch

I have a 2001 Pontiac Sunfire manual transmission
that does not have the rubber piece on the clutch
rezivore cap that seals it so when u push the clutch
it just falls to the floor and all the air goes up
through the cap. If I get the rubber piece will it seal
it up enough for my clutch to work as it should or is
it gonna be something bigger like the slave or
master cylinder. So since there is no compression I
had to float gears to move my vehicle and I down.
Shifted from 5th to 4th then tried to put it in neutral
but it stayed in 4th gear. I could actually put it in
1st 2nd and 3rd gear but it still drove like it was in
fourth. What's the problem and how do I fix it.