1980 MC9 Crusader II - Air leak under the entry steps (bifold door)

[bluesblake]

Hello,  My bus will not get over 80lbs of air pressure .  I have an air leak under the entry door.  It seems there is a threaded Air valve that should have a cap on it, maybe a metal acorn cap or something.   The air coming out of it is substantial. 
Does anyone have any ideas? Do I just need a cap for that threaded valve and if so where can I get one? 
Thank you ,   Beginner bus guy here, we have had it for 2 years and are yet to take a real trip in it.  Hoping to get the items tightened up and get on the road.