Ran into this a few weeks back. If you have received a Melling 57HV (big block Ford FE 352-390-406-427-428) oil pump, look at the plug that holds the pressure relief parts in. IF it has a hole in the end, that hole will suck air in. It is a defective oil pump. I got this from a posting on the Ford FE forum.
The good plug is blank - NO hole.
The pump I returned to Rock Auto had this hole and had a date code of July 2022, the box stated "made in Mexico".
Melling did not give me any date range of defective pumps.
Just posting this so if you are having issues, look there first. Luckily for me, I was not past the part where this would have been inside the engine already.
By the way, this pump at this time has been dropped off the list of available pumps at Rock Auto. Good move eh?
I find this an interesting issue in that I have an NOS Ford FE oil pump with the hole in the plug and another aftermarket pump from an old rebuild that has one as well. But I do have a new Melling M-57HV that was purchased several years back that does not have the hole.
I always get the Melling pumps (various depending on the wants of my customer) from Precision Oil Pumps. That way they are blueprinted and will always work perfectly. The cost is well worth it.
October 31, 2022
Since this pump in now back on Rock Auto's listings, it might be safe to say to is OK to buy this pump again.
Those made around June/ July 2022 are the suspect pumps.
Just had the same issue with one of these pumps on a 428CJ build! So yes, inspect them first!
We just ran into this as well. It was the standard Melling 57 so looks like not just HV. This is why we always prime and check engines in run stand. We found during priming so motor was never started thank goodness