Fuel flow diagram for a 1975 300D?
 

It would be helpful for me to figure out where all these hoses go and where these bubbles keep coming from.
Can anyone point me to a good resource?

It would be helpful to know where you are seeing the bubbles and if the bubbles are moving.

I don't know where there is pictures of the whole Fuel system but will attempt to describe it.

There is a Fuel Supply System and a Fuel Injection System.

Fuel Supply System: Stats with the Fuel Tank, Fuel Flows through the Fuel Tank screen into hoses under the left side of the Car on a W123.

Below is the most likely pace where Air is getting in to make bubbles:
The hose comes out on the left Fender Well and goes into the Plastic Filter (primary Filter) and into the Fuel Supply/Lift Pump. Since the Fuel Supply/Lift Pump is supplying the suction on that side of the pump which is the Fuel inlets side this is were there is a chace for Air to be sucked into the Fuel Supply System. An alternative area for Air to get sucked into the system is the Hand Primer Pump.
The more forward Hose or Tubing that comes out of the Fuel Supply Lift Pump (has fuel that is under pressure) goes to the Spin-on secondary Filter. From the Secondary Filter the Fuel goes into the Housing of the Fuel Injection Pump where it is available for the Elements (that supply the Fuel high pressure Fuel Injection to the Injectors).

The Fuel in the Fuel Injection Pump Housing flows through the Housing past the Fuel Pressure Relief/Overflow Valve that controls the Fuel Supply Pressure in the Fuel Injection Pump Housing.

From the Fuel Pressure Relief/Overflow Valve the Fuel is exiting the Fuel Injection Pump and is on the way to the Spin-on Filter Housing and out the Cigar Hose to the tubing on the Fender well and under the left side of the Car and back to the Fuel Tank.
That excess Fuel that ends up back in the Fuel Tank was used to cool the Fuel Injection Pump.

The Elements one per each cylinder supply the High Pressure Fuel to the Injectors and are not the cause of bubbles.

Thanks! Lots of nice data.
The first bubbles I see are from what I think is the lift pump in the clear hose going to the primer pump. I just replaced the hoses from the fender well to the clear filter and from there to the lift pump.
It's a W-115, by the way.
And then there's that hand pump. It doesn't always feel like it's doing much.
Sounds like this weekend's chore should be rechecking my hose and filter work. Maybe I put a hose on upside down or something.

Be aware that the primer pump often leaks and is a very common source of air ingress. Poke around this forum for more info, or I'd imagine someone more familiar with the 61x series engines will chime in soon with what to look for.

Look at he pic in post #21 of this thread and tell me if you have the Hand Primer and Filter Housing that is shown in the pic. The red arrow points to the hand primer which has a large black button on it.

http://www.peachparts.com/shopforum/diesel-discussion/339643-1976-300d-injection-pump-delima-2.html

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Mine looks a lot like that. I'll take a better look in the daylight tomorrow.

black primer button
 

If your primer button does not feel it is getting harder to push down and you do not hear a "buzzing sound" after you push for a while (coming from the overflow valve on the back of the IP) then you have an air lock on the top of the secondary fuel filter. Just loosen the large bolt (22 mm) that is in the middle of the metal fuel filter to the left of the primer button. When you start priming again you will get all kinds of bubbles and fuel. When you only get fuel tighten the bolt back down. That is an aluminum housing so don't overtighten. A small bubble in the prefilter is ok. Depending on whether you tried to start the car while it was airlocked you may or may not have to crack the injector lines to help get the air out of those as well.

Can you give him a description of the fuel flow from where the fuel line comes off of the fender well.
The description I gave was for my 1984 300D and it may be different.

The only difference from Diesel911's description would be the location of the hand primer. After the lift pump the clear line goes up to the hand primer which is attached to the secondary filter.

There is another spot on the W115 that should be checked for leaks or old hoses: On the fuel tank there is what is called a "stepped hose" , it is roughly 3" long and transitions the fuel tank to the steel fuel line. It has two ID's: one side is 8mm and the other is 10mm. If this is showing any sign of wetness it needs to be replaced as it is often overlooked. They are available on this website. Part # 1114761526

Oh yeah, I did that stepped line last fall. It wasn't leaking when I brought the car home but it started to when I touched it.
This car sat for ten years. It all looks great but it's a little crunchy here and there.