I'll bet you've got an air leak at any of the six or seven joints that use o-rings. The o-rings in that car are a known weak point (most or all of the connections between the injection pump and the fuel filter housing use these o-rings). Standard procedure for me is to replace ALL of those o-rings if even one appears to be leaking, because they can cause so much trouble including exactly what you are describing (loss of power and shop can't bleed the air out of the system).

The intake manifold will need to come off to do this work. If money is not too tight, and your old fuel lines are no longer clear (the lines between injection pump and the fuel filter holder) then in addition to all the o-rings, replace all those clear lines.

The injection pump and fuel holder needs to be really clean and stay clean during the work, so there is no contamination of the fuel system with dust or grit, which can damage the injection pump and the fuel injectors.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC