What happens to the air when a diesel fuel filter is pumped?

Technology changes all the time, but as of today you have older technology diesels with injector pumps and newer cdi technology with high-pressure pump, common fuel rail and electronic injectors.
Powerstrokes use oil pressure to compress fuel in each individual injector, but with electronic injectors the principals are close to cdi.
Only very old injector pumps had priming pump that could be hand-operated to bleed the air from low-pressure lines and filter.
For bleeding air from high-pressure line you would crack nut on steel tubing at the injector.
On rail injection system usually you have electric priming pump and bleeder on filter.
You open the bleeder and turn ignition on without activating the starter, what turns electric pump for few seconds.
Once you see fuel coming from bleeding port, you close it and let the high pressure pump to push reminding air via injectors.
Lot of injectors don't like air in them so beware of running diesel dry as some Ford mechanics say it can cost you $4000 in piezoelectronic injectors replacement.

Many filters also incorporate a bleeder screw. You loosen it and pump until air bubbles stop coming out. Then, tighten the screw. The downside is that a little bit of fuel comes out but you just clean that up with a rag.

Thanks Brett - that's what I though happened, but I was always worried if I had air problems that I'd have to get into the engine compartment and figure out how to crack an injector.

Thanks Lanny - that's an interesting article and it gave me good contact info for Racor's tech people. I'm thinking of modifying my fuel filter system. If what I want to do is a good idea, I'll post the results later.

Bill

The air is forced through the fuel return line to the tank.