I see that this conversation is quite old and quite a bit has been said on the subject.
I have just completed a BSA A10 Super Rocket rebuild and thought i would share my experience.
Despite my best efforts and lots of attention to detail, my rebuild initially resulted in a really bad case of wet sumping and serious build up of sump pressure, and i decided to split the motor and check to sort it once and for all. i had followed the manual religiously and had obvioulsy missed some thing important.
1) The anti flow back value is located behind the delivery side of the oil pump. This valve is a small ball and spring, located by a grub screw. This is accessed from the inside of the crank case, just next to the timig side main bearing. That is right guys, split the motor to get to it!
Now the part that took me by surprise. I find absolutely nothing on this in the Haynes manual, (only a pic in the parts list). So for those rebuilding a motor, watch out, this part is easily missed and as i could not find any mention in the manual, i expect others may also miss this point. (Ok yeah right, i'm no expert on A10 motors, so had to find out the hard way).
2) So the rest seems quite obvious, if you want to stop wet sumping, this is the right place to do it (with the motor split) drill out the punch marks that retain the grub screw (probably using a 3 to 4 mm drill bit, and don't penetrate teep, just a fraction deeper than the punch mark it self). Once the punch marks are drilled, the grub screw should come our quite easily, revealing a spring and small metal ball (valve). The spring and ball should live in a world of oil in a healthy running engine, but if the machine has stood or has been stripped, you may find a ball of rust, grime, dirt or gasket compoind in the valve cavity.
3) Restoring this part is real easy, several ways are mentioned. (assuming you have cleaned the years of dirt and grease away and the casing clean up process is behind you). I used a new ball and new spring (note that this is a really light duty spring, much smaller than the spring used in the pressure relief valve). Locate the ball, use a suitably sized flat headed pin punch and 1 pound hammer and give it a smart tap, to establish a sealing seat for the ball. Now for a quick proof test. Locate the ball, spring and seat in the retaining grub screw. Now from the oil pump side (outer side) of the casing, you should be able to blow (lip contact required, yes guys, just your mouth!) through the oil pump delivery hole, and the valve should hold light pressure. If you blow hard, you will actually be able to lift the ball off its seat. it will rumble as the ball lifts and seats itslef rapidly whilst you blow. If that works OK, then give it a light suck, and cover the hole with your tongue. If the seat is good the vacuum you cause (with the mouth suck), will be sealed in with your tongue. I had this hold for about a minute, before I broke the vacuum my slef.
For the technical boffins with workshop facilities, i expect some tool or tester may be avaialble, but for me (the home restorer)this worked quite fine. I really have nothing against getting up close and personal with my BESA for some tongue action!
4) If the above process works out OK, then you can permanently return the ball, spring and grub screw. Your choice on how to securethe grub screw safely and permanently. The screw should be screwed in tight and locked. I prefer using lock tight thread lock compound. (cant stand the hammer and punch method my self).
After following these few steps, i assembled the motor and so far so good. No wet sumping, and good oil pressure and flow back to the oil tank.