Hydraulic lifter troubles

[Footprint]

CU 2.0l aircool, 1982.

In the past my van has occasionally spent a few days or sometimes a week unused, as expected when starting there would be a rustle for a few seconds, or if unused for a longer period perhaps 30-40 seconds, and then the lifters would pump up and normal service would be resumed.
Recently however I have been experiencing troubles, every time I start the van - even if it's has been used the day before - there is a horrible knocking from the lifters which lasts a number of minutes and then very gradually reduces, sometimes taking half an hour before everything is happy again.
Oil changes are regular, (the last one was only about 500 miles ago), the sludge trap in the strainer never has much in it, so I don't think the engine is particularly mucky inside.

Clearly something isn't right, and I shall have to get the sods out and have a poke around, obviously the usual cleaning and checking will have to be done, but unlike the one previous problem I have experienced, (a freaky silent lifter failure, leading to running on three), this seems quite random, by which I mean it isn't always the same lifter that takes a while to fill, obviously I'm not certain which is empty at any given time, but sometimes it's on the left, sometimes on the right, leading me to think perhaps the lifters themselves are not the fault, and perhaps there is another cause.

Any thoughts, or possible things to check?

[jamesc76]

im sure you can get solid lifters for the aircold engines ???? perhaps fitting a set of them instead ?

[kevtherev]

Lifters draining would suggest a few things...

That the return spring within the lifter is weak.
Wrong oil grade.
Poor oil pressure.
Lifter preload needs resetting.

For what they cost and relative ease of fitting I would be renewing them
and checking my oil pressure with a wet gauge
and my oil temp after warm up.