I've had several different Milidon external oil systems loose the prime after sitting a short while, 8 hours upto 5 days, others that never lost the prime after I started making my own oil pump to block gaskets that fit around the oil hole to the block and snug against the boss that the oil pump drive fits into.
As far as the O ring on the boss, snout, it shouldn't slide up into the block easily, even with oil or grease on it, it should have to be work and have to be twisted around a little while pushing it into the block