5.3 engine noise

This engine has 140,000 on it. However I am the 2nd owner and it has always been serviced mechanic has told me a lifter issue. However without the valve cover they are all pumping. I blew out all the push rods with air and also put them back in and blew through rod to the lifter. All seemed to be open. It took about 50 seconds to get the oil to the top of the lifters this morning at start-up. The pressure gauge seems to be pumping 40 . Could the oil pump be weak? I’m fairly mechanical, but hav