Since you have the engine on a stand---------remove all the pushrods so no oil will go to the top of the heads and use your prime tool to pump the oil to see if you get the same results-----if you don't you will have identified that its an issue of the heads not draining back (since the heads weren't changed)


Then, you can remove the back two lifters so all the oil is from the lifter galleys is dumped right back down into the lower end---------and you should maintain pressure as the oil will be on a very short circuit to return to the sump