It's a positive displacement type pump, which means it moves a fixed volume with each stroke and the pressure will rise as high as necessary to let that volume flow. With no resistance to flow there won't be any pressure. With the lines sealed, the injectors block the flow until a certain pressure point is reached, then they "pop" and let the fuel out.

Turbo?

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

I used to think the same. There is a thread that go deep into the subject of the positive displacement. I believe it was Numbercruncher that disproved that.
The argument over that was hot enough that ForcedInducton got banned.

Apparently the Piston is pushed positively in one direction against Spring Pressure but on the way back the Spring Pushes the Piston. This means that that if the Outlet of the Fuel Supply/Lift Pump was Plugged the Piston being pushed back under Spring Tension would not move until what is blocking the Outlet is removed.

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84 300D, 82 Volvo 244Gl Diesel

I see, so the pressure can never rise higher than what the spring tension can produce? That makes a lot of sense. Otherwise one clogged injector would likely destroy the pump.

The Injection System is separate from the Fuel Supply System.

In the Fuel Supply System the Fuel goes into the Fuel Supply/Lift Pump passes through the Secondary Spin-on Fuel filter goes into the Fuel Injection pump housing (this is where it supplies Fuel to the Elements that create the high pressure for Fuel injection) and passes through the Fuel Pressure/Bypass Valve (that controls the pressure of Fuel Supply System) and from there ends up back in the Fuel Tank.

If the Secondary Spin-on Filter became entirely plugged the Spring Loaded Piston of the Fuel Supply Pump would protect something from popping. But, of couse the Engine would also stop if the Secondary Filter was entirely plugged.

The Fuel Injection pressure build up in the Elements, Hard Lines and Injectors and there is no sort of relief valve (in a sense the Injector itself is the relief valve) in that part of the system to deal with any blockage.

And, in fact I have twice seen where a Mechanic managed to get something inside of the Fuel Injection Hard Lines that blocked the Injectors.

Both time it cracked the Fuel Injection Hard Lines.
In one case the Hard Line cracked in 2 pieces.

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84 300D, 82 Volvo 244Gl Diesel