One last thing I have just thought of it to make sure the lever on the lift pump is at the lowest point on the engine camshaft when you use the hand primer.
This is so that you get a full stroke on the diaphragm to deliver the maximum amount of fuel. You can tell what position it is in by the resistance on the hand primer.

Yes, I'm aware of that, but as mentioned before after a couple of "pumps" the pressure builds up and the diaphragm/membrane doesn't return with the lever so that you can feel it's not actually pumping much. Opening the pipe connection into the injector pump releases the pressure and the diaphragm/membrane returns. That's why I suspect some kind of blockage, and the filter screen is a great candidate.

I don't think that you should be able to pump fuel through the injection pump (other than through the pressure-relief valve which might vent back to the fuel tank). The injection pump controls the squirts to the injectors.

The MK relief valve is set to 20 psi (the electric supply pump keeps the supply system at this pressure - the pump delivers fuel up to 25 psi, and the excess 'spills' back into the fuel tank, together with the leakage from the (in-line) injection pump gallery).

As I see it, I shouldn't be able to pump through the injection pump with the venting screws closed, but I'm talking about when they are open. Pressure still builds up when they are open, and only very small amounts of fuel comes out when bleeding. That, combined with the fact that the lift pump stops "gripping" after a couple of pumps (if I wait a while, pressure is reduced enough to let me do another pump) indicates to me that somewhere the fuel has a hard time getting through. Since loosening the incoming pipe to the injector pump releases this pressure, it's now narrowed down to either being the filter screen (hopefully) mounted on the inlet to the injection pump, or somewhere inside the injection pump.

Unfortunately the weather gods have decided to make it rain here today, so I'm not going to check this until tomorrow at the earliest (depending on the weather).

Just to add to your comments above, a stuck stop control (in the closed position) would stop any fuel from passing through.

Check the control cable or the solenoid wiring if the stop control is electrically operated.
(I think that this has already been mentioned above)

Yes, it's already been mentioned, and I as I've stated it moves freely outside the injection pump. What happens internally however is unknown, and I consider that as an alternative explanation, but I'm not sure it helps me if that is the case since it doesn't seem to be much I can do about that without opening the injection pump, and that isn't really an alternative. I can't even understand how I could get enough space to manage to remove the injection pump, and opening them without detailed drawings/procedures has little chance of success. I did manage to unstick a stuck stop valve in a Ford D-series "Minimec" injection pump once, but I promised myself to never open one again - I wasn't at all sure that pump would ever work again (but it did actually). But this truck hadn't been used for several years, this is not the case with my Bedford and I can't really believe that the stop valve would have the time to just "get stuck" from corrotion, so it would probably mean some kind of internal mechanical failure.

I'm still hoping for a "simple fix" like a blocked filter screen.

The cab tunnel might have a removable panel (some TKs did have) which might allow you better access without removing the cab.

Alternatively, access from an inspection pit (assuming you can move the vehicle!) might be possible. My memory is vague (more than 30 years since I was last 'under' a TK).

The major drawback when removing the cab is whether the mounting bolts (which are through-bolts with nuts - ie they don't screw into the mountings) are seized (despite being copper-plated).

If you do decide to remove the cab I can provide details of what needs to be disconnected. The major thing is that to lift the cab you must use a beam through the door apertures (not the window apertures), otherwise the cab will distort.

There are nothing more to remove I'm afraid, I've welded quite a bit on the cab (most of the floor was gone), so I have a pretty good idea about what's what there. IF I at some stage have to remove the cab, I will definitly need all the tips I can get about disconnecting everything, but I'm a bit worried because of rust around the edges of the roof, especially in the rear. If the cab would "bend" enough to break the windshield, it would be a catastrophe.

One thing is sure: If I ever do remove the cab, I will replace the whole engine and not just the injector pump! The engine is as mentioned before, quite worn even though it's been running fine until now. It's oil consumption however isn't what you would call environmently-friendly ;)

The weather allowed me check the filter screen today, and there was one. It was however relativley clean. I did find some kind of a valve under it though, with a small pison itside that was stuck in some "brown stuff" - not rust as far as I can tell, but some really sticky brown stuff. I got the piston out and cleaned it so it now moves freely again, and put it all back together. I had some hopes after that, but the result is the same. Bad flow when bleeding, and no pressure out on the injector pipes.

I don't know what that "brown" stuff I keep finding is, but it's from 1971 and I guess it's had it's share of bad/dirty fuel along the line. Since this can pass through diesel filters and the filter screen, I'm guessing it's something that's solvable in diesel. There's no telling though which other parts inside the injector pump that could've gotten stuck by the same stuff - so at this point I consider that the most likely culprit. However I see it, I must conclude that the problem is inside the injection pump.

Can any of you tell me where the timing adjustment for the injector pump is? It seems to be 3 bolts that hold the injection pump, and what I'm wondering is: If I remove these (and the injector pipes), will I also loose my timing setting? I'd hate to disturb the timing, since I have no proper means of readjusting it. If I could get the injection pump off without loosing the timing, I guess it could be worth a try to sit down and remove any inspection covers and plugs that I can find and clean and "unstick" anything I can access without pulling the pump completely apart.

The following information is offered without guarantee (my knowledge is chassis, not engine).

According to the MK manual (which is for an inline Minimec pump, not a CAV rotary pump), there is a 'coupling' (like a dog-clutch), probably with a rubber insert.

Provided that you do not disturb the pinch-bolts that secure the halves of the coupling, you should be able to remove the injection pump and replace it without disturbing the timing.

That is how I read the manual which describes how to adjust the timing:-

So, don't disturb the timing and you should be OK.

Edited to add:- read the next post before proceeding to remove the pump.