The effects of an air leak into the fuel system might cause you more noticeable difficulty at lower rpms as well. . Have a look at the prefilter for a stream of bubbles going through at idle. The constant bubble always present is normal. A stream of small bubbles is not.

Also those older hand primer pumps can also be suspect. Just stretching a balloon over the white handle onto the main cylinder casing and squeezing the air out of the balloon can eliminate that part temporarily from the equation.

Sometimes better than replacing something on spec and even cheaper as well. Another cheap test is to close off the return line from the injection pump. This allows the fuel pressure in the pump to rise.

There should always be a noticeable difference I now suspect. If none at all the feed pressure to the injection pump is lower than it should be. Chances are it is too low to even open the return valve.

In a fairly normal car the clamping off is going to elevate the internal injection pump pressure. So you would hear the engine smooth out somewhat or something else.

The design of the lift pump does allow total restriction of the return line and does no damage to the lift pump. Anytime there is low power on one of these old diesels it is a good proccedure to try in my opinion. Again it is cost effective and does indicate something.

You should ask on site about interpretation of what changes if any you observe though. Some will be normal and some will not be. This is a matter of judgement or at least some of the possible symptoms will be.