Question 1
when oil is hot oil pressure goes from 4 psi to 1 psi when I brake hard it is a known problem

Question 2
oil capacities
operator`s manual writes 5 (qts.) for 272 light duty and 6 (qts.) for 272 hevy duty, the 5 (qts.) I have filled do not come to top mark on oil dipstick
is it the pan or dipstick different from light and hevy

Question 3
have bought a new oil pump screen at Carpenter the small valve what function it has

The oil pan needs a baffle to prevent oil slosh to the front when braking.  Some year models of rear sump pans came with them while others did not.  Here are some past threads on the subject.
http://forums.y-blocksforever.com/Topic149135-1.aspx
http://forums.y-blocksforever.com/FindPost144277.aspx
http://forums.y-blocksforever.com/FindPost74375.aspx 
  
Regarding oil pan capacities, you will need to add a quart for the filter.  It’s six quarts with filter change for the pickups and seven quarts with filter change for the HD engines.  That ‘valve’ in the side of the oil pickup screen is a safety valve that opens in the event the screen gets completely stopped up.  This allows the oil pump to continue pull in oil although at that point the oil will not be screened at all until it gets to the oil filter.

Lorena, Texas (South of Waco)

Thanks Ted for the quick answer I will start with and refill 1 (qts.) More and if that doesn't solve the problem I will build a baffle I know there is no one in the pan