Down the road you will get to a point where you can get many of the spool pins to set without setting the non-spool pin at all - get them to set in their true binding order, but it takes some time to get that level of feel. In the mean time just keep setting the pin that keeps falling, like others have said. You may also want to give this lock a try. It absolutely screams out which pin is the security pin and is super-easy to pick, but the actual action of the pins is much smoother and less gritty than the Master 140. If you continue to have trouble with the Master 140 it might help.
My first spool lock was a Brinks 161 from amazon. It has at least two shallow spools in it. I get a false set and small counter rotation.