The HPOP reservoir is fed from the drain back valve on initial/cold starts, but like mack 1 said the LPOP is what feeds the drain back valve. If the oil gauge on the dash is taking longer than 3 seconds to rise to the normal level it is a sign that the LPOP is not up to snuff.
Here is a recent thread on the valve which if it has debris in it can cause the reservoir to leak down overnight. If you check the level prior to cranking the first time of the day and it is low look to the valve. If the reservoir is full and empties while cranking look to the LPOP.