My friend is having a frustrating problem with his 82 diesel Westie
that has a 2.0 GTI engine in it. The engine is mounted on the stock
diesel mounts.

After it sits for a week, the oil pressure will not come up for more
than 30 seconds. It's not the sensor, the lifters rattle.
Removing the oil filter mount and priming the pump makes it work again.
For some reason the oil pump seems to be losing its prime.

It has a long repair history,
1) Blown head gasket, head resurfaced and new gasket.
2) Soon after, Oil in coolant, leaking oil cooler. replaced with used
one off another diesel.
3) Gradually (over 3 months) takes longer for oil pressure to come up.
Replaced pump with a new rabbit one and pickup tube with a new diesel one.
4) Problem just keeps getting worse. 30 seconds to get pressure after
sitting for just 2 hours.
5) Replaced engine with a fresh used one. Transfer oil pump, pickup
and pan from the troubled engine. Seems to work fine, then sits for
two weeks, no pressure. AArgh!!  Manually prime pump and pressure
comes up. Once the pump is primed it has plenty of pressure.

Needless to say, he's pretty frustrated with it. I'm stumped as well.
Does anyone know what keeps a oil pump primed when the engine is off?
Is there anything that one could screw up when replacing the head,
oil pump, or oil cooler that might cause it to drain down?

TIA
  -Otmar-
82/86 Stretch Vanagon Westfalia Syncro GTI. "Power of two"
http://www.evcl.com/vw
Several EVs (Electric Vehicles)