I have a problem with the oil prime on the 3.5 v-8 in my 1976 military 101 Forward Control. For a while it became progressively more difficult to get engine oil pressure at start up. (The engine has under 10,000 original miles from new, no reason to expect anything to be worn out. I have both an electric pressure gauge and no pressure light so it is unlikely both are malfunctioning and I confirmed no oil flow). Eventually the oil pump would not prime at all. The pressure release valve seems free, not hung up, no step or wear issue I can feel. The oil filter is the proper anti back flow type and I changed it (and pre-filled with oil) to be sure it was not defective. Oil pan oil level is fine. Still no pressure. I tried priming by pouring oil down a hose into one of the gauge ports in the pump before cranking with the coil wire removed. No pressure I tried priming by removing the distributor, machining a shaft to engage the oil pump and turning the pump with an electric drill for 15 minutes. No pressure. I pulled the pan and checked the oil pick up strainer. Looks clean. Oil pan sludge free. I then bought two brand new gears for the pump and installed them packed with vaseline. No pressure. Repacked it again. Still no pressure. I am running out of ideas. I guess I'll pull the pan to see if the oil pickup into the pan is cracked or has come loose where it attaches. The official 101 parts manual doesn't even show the pickup part, (hard to figure that one other than having been created in the Leyland era corporate ownership ) but the 3.5 range rover parts manual which is the same basic engine shows a slightly different looking pick up (round rather than rectangular strainer) that bolts on with a gasket so I suppose it could be sucking air at that gasket or join. Any other suggestions from anybody???

 

On a non Landy 70s era car with 200k miles plus - The distributor drove the oil pump. The oil pump seized, a shear pin sheared and the engine kept running without any oil pressure. I know nothing about your engine but I wonder if your oil pump design is similar.

Years later on another car, it too lost oil pressure. I dropped the pan and discovered a nylon plastic baffel near the intake got brittle, broken into pieces and clogged the intake. I can't imagine your engine having any plastic in the oil pan though.

Good luck.