oil pressure problems

just fitted the engine from my dads old discovery in my 110. got the engine in and all coupled up. when i started it up the oil light stayed on. swapped the pressure switches over and still on so took off an oil cooler pipe and it was empty. when i changed the engine i had the filter off and it has filled again. i let it tick over for about 5 or 10 minutes but still no pressure. any ideas?

200 tdi ?,right wire on pressure sender ,oil filter housing has thermostatic bulb so oil is only circulated through cooler when hot

got it sorted was just air somewhere checked i was getting oil to top end then let it run for about 20 minutes and the light is behaving now.