When is a fuel pump really dead, spins but no pressure Options

[swl]

Yah - I know " when it doesn't pump fuel"... but I'm desparately trying to save the cost of a new pump.

73 914 1.7

The fuel pump was totally siezed from sitting with gas in it for a couple decades. I was working fine before the coma. We pulled the pump and removed the housing. As you would imagine, the spinner thingy and the roller bearings were all gummed up. Little bit of car cleaner and elbow grease cleaned it up just fine. Reassembled, bench tested ok - ie motor spun and there was a little splash from fuel down in the body of the pump.

We tested outside with a small container of gas and only the supply and discharge lines hooked up. It wouldn't pump. In fact there were bubbles coming out of the supply line. We kinda scratched our heads and decided to put it in the car anyway in case it had something to do with priming. Still no joy. BTW we completely rebuilt the lower fuel lines, cleaned the tank, replaced the strainer sock ... Just to be sure we pulled off the supply hose. It didn't gush immediately but as we lowered it we got a good rush of gas. Makes sense - the rear end is jacked up and there is only about a gallon of gas in the tank.

Oh yes - one other oddity. When it was in the car we broke the fuel return fuel line where it joins the plastic hose going back under the car. When we did this we got bubbles in the fuel tank.

So some questions:
Does the system need gravity feed for priming the pump? If it does then lowering the car may get it going.

There is a pressure valve between the D and R ports. In it's "at rest" state it blocks D and opens R. If you push it outwards (toward the crimp cap) D opens up and R closes. Is this the way it is supposed to work? (keep the lines pressurized when the pump pressure goes down?)

Can you point me to some place that would explain how that type of pump actually works? I can't for the life of me figure out how spinning roller bearing can create fuel pressure?

Is there anything else we can try before biting the bullet and buying a new pump?

Thanks
Steve