Setting the air in the accumulator is a balancing act. If you put too much in, you'll have plenty of air pressure to push
oil back to the engine when needed, but there won't be as much
oil to push. If you put too little air in, there will be plenty of
oil to push back, but the pressure to push it will bleed off very quickly.
I just reread the Accusump instructions for setting the precharge. It says to open the valve with the engine off and put 60 PSI in the accumulator. That will push the piston all the way out. Then bleed the air pressure down to 7-10 PSI. That is the correct precharge. ... If you do the math for an engine running at 50 PSI of oil pressure, that will result in the accumulator holding about 80% oil and 20% air. It also means the air pressure will have dropped by half by the time 1/4th of that oil has been pushed back into the engine in a low oil pressure situation. I set my 3 quart accumulator up with a precharge of about 15 PSI. That means my accumulator has about 2 quarts of oil in it at about 50 PSI and it will push one quart of that back before the air pressure drops to 25 PSI. If the engine oil pressure fails completely, the accumulator will push the last of its oil back out at about 15 PSI.