??? Is the return into the tank at the same height as the outlet from the tank?

No. The supply is through the in tank filter from the bottom of the tank. The return is above the bottom of the tank, I am not sure of the distance, someone said 1/4 inch. Sounds a little close to me.

Another thing you can do is crimp the vent line and disconnect the supply and return lines in the engine compartment and pressurize the return line. Fuel will shoot out the supply line.

I finally got around to replacing the fuel lines at the tank on the 77 300d today.
Here's how it went.

I went to the auto parts store and bought 10' of 5/16 hose.
I'd use it for draining and replacement.
I backed the car's rear wheels up onto ramps.

I disconnected the return line at the secondary filter and put the 10' 5/16 in it's place.
The fuel gauge read 1/4 full.
I put the other end of the 10' piece in a 5 gallon fuel container and started the car.

While drinking a cup of coffee on the porch, the engine slowly emptied the fuel tank into the container.
After about 1/2 hr at idle the 5 gallon container was getting full.
I then backed up the 85 TD right next to the 300d and fed the 10' line into the TD's fuel tank and let it run for about another half hour before the engine stalled.

I figure I drained close to 10 gallons out of the tank.
So much for an accurate fuel gauge.

When I pulled the outlet line off the tank about 1/16th of a cup ran out on the ground.
I then replaced all the rubber fuel lines at the tank and with the car still on ramps, put the 5 gallons back into the tank.
I took the opportunity to replace the primary filter and thought that since the car was on ramps, it might gravity feed to the primary filter.
No flow at the filter.
Looked like I was going to have to prime.

But before I did, I thought I'd try to start it.
Fired on the first compression stroke and ran starved for fuel for about a minute.
Wouldn't rev up but would keep idling.
After about a minute, it returned to full fuel flow and off I went.

Much better way to drain the tank than to have fuel running down your arm as you try to drain it under the car.

Sounds too easy, I vote against it due to Shade Tree Mechanic's Creed. ;) :thumbsup:

Efficiency is just thoughtful laziness.

............leave it to a philosophy professor to come up the the better way...............:o