A accumulator only requires one line. No "in" and "out". A few years ago, I convinced Doug Garifo to make some filter adapters with a large 1/2" pipe thread in the passage that feeds the main galley...right next to the 1/4" pipe that picks up oil pressure. Before that, we would drill the housing and weld a #10 fitting to the adapter. From the beginning, I used only a Motorcraft filter BECAUSE there is a drain back valve, which I used/still use for a check valve to prevent backflowing the filter element. It worked perfectly, and has worked perfectly for at least fifteen years on every FE we have put an accumulator on. Wonderful product. I prefer the three quart Moroso. More is better. BTW, BB...you are correct....25 psi in an empty 3 qt unit will hold 2.5 qts of oil with a 70-75 psi max engine oil pressure. The compressed air does offset some of the capacity, and it varies depending on max oil pressure and initial air pressure in the empty accumulator. I think it needs at least 20 psi air pressure with no oil in it. It has to have enough force to do its job. Using less air just to make it hold more oil will not be a good answer. They absolutely work............no doubt about it.