If you can only get 25 psi with a drill you got a problem in my opinion. Heck I primed my last eng by hand as I used a 3/8 ratchet with a 5/16 socket on my oil primer rod and turning it by hand I had up to 50 psi with no problem. Unless it a real weak air drill that wont turn much against the resistance of the oil and the pressure you should be able to get over 25 psi with a drill as long as the drill is strong enough as most electric drills are strong enough. Sure the eng turns faster but like I said on a fresh built eng I can always get way past 25 psi using hand tools. I remember well as I was curious how much I could get like that and thats why I tried it by hand and ratchet. And I did it all by hand as I did not use a drill at all I just turned the oil priming rod with the ratchet from the start of priming and got it up to 50 with no problem.
One reason I did this was because some years back I built a 460 for my buddies Mustang and it would only pull about 20 psi priming with a drill and I figured it was ok and would pull more psi running as it was a fresh build. I was wrong as it would only go to 25 to 30 psi tops even at 3000 to 4000 rpm. His buddy told him it was fine and I said no way and pulled the eng back out of the car. I ended up finding Launiti had the roller lifter instuctions wrong as the roller lifters were in with the oil hole in the lifter facing the galley oil hole in the block and it bled to much pressure up thru the pushrods to the rockers. I had to turn the lifters around and put the oil hole away from the galley oil hole and then 60 psi no problem. I could also get 50 psi by hand with that eng also when I fixed it. Ron