I agree with the rest if you are seeing Air Bubbles you have an Air Leak. You actuall diagnosed the problem yourself. (Why I never take anything to a Mechanic; the think they know what they are doing but don't.)

The reason you do not see Fuel leaks when you have an Air leak is because there is 14.7psi of atmospheric pressure at sea level pushing on the hoses and such all the time and Fuel is thicker than Air.

Combine the 14.7psi pushing in and the negative pressure inside when the Fuel Supply/Lift Pump is operating and you get an Air Leak through areas Fuel will not come out of.

You can change the 2 sections of Fuel Hose that hook to the Plastic Primary Filter for a few dollars with SAE 5/16 30R7 Fuel Hose that sells at any Auto Parts store. It is rated for Diesel Fuel as well as Gas.

Look under the Car at the Hoses that come out of the Fuel Tank and see if they look old and cracked. Also this is the place you might see the Hoses seeping as it is the lowes place in the Fuel System. Or you could simply drain you Fuel Tank and change the Hoses.

All the Hose changing is way cheaper and less complicated than the IP replacement you had in mind.

The Hand Primer is the next likely source of an air leak.

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84 300D, 82 Volvo 244Gl Diesel