300SD Fuel Issues

[Rocketrider]

I'm having an issue with my 82 300SD and now it's time to consult with the Grand Poobas! I bought the car with 152K miles and a bad tranny for $450. I just happened to have a parts car of same model with a bad engine and good tranny. The car had been sitting for almost two years, with very little history before that. I replaced the tranny and now that I'm driving the car, I have discovered several issues.

The engine ran great when I first drove it with 1/4 tank. Then I filled it up and it ran great until I got to 1/2 tank. Then it started acting like I was running out of fuel. It spits and sputters, loses power, and eventually dies. It gets worse and then won't do anything other than idle at 3/8 tank or less. I fill it up and it runs great until it hits 1/2 tank again.

Now here's the kicker! When I was left stranded, I pumped the primer at the lift pump and had lots of air in the lines. The engine cranked immediately and ran great for a quarter mile or so, then it died again. Pumped the primer again and the engine cranked immediately. I finally got the car home and now the primer doesn't work at all. It takes ZERO effort to actuate the primer and it never gets firm. Moreover, it is leaking fuel around the shaft of the primer. Obviously it needs to be replaced and is leaking air into my fuel system.

Now, is it possible that the bad primer is what was causing the engine to run like crap below 1/2 tank? I'm told that above 1/2 tank the fuel level is higher than the lift pump, which would explain why it ran so well. Is it possible that I have a bad lift pump as well?????