Wow, who is selling parts for this IP suction/fuel return pump? Fastlane? I've suspected my 616 IP pump is not sucking very well, I've run out of fuel frequently with fuel in the tank, the last thing I did was blow the two lines to the tank out with air, remove the fuel tank and scape the quarter pound of gritty dirt stuck to the bottom of it out. I've been thinking of putting an electric booster pump in the tank line, but replacing the IP suction pump check valves would be a much better idea. I would really suspect that 189000 miles would lead to leaky check valves. Since you seem to be experts here, any easy to get at high maintenance items also associated with the IP pump? You've sold me on the cigar hose, I had no idea what one was and they are cheap.
My Fiat tractor with the similar Bosch patent pump is a no-start, I've been putting off rebuilding the engine, but putting check valves in the IP may be my problem on this 7000 hour plus engine since it mysteriously stopped pumping fuel out the injector lines. (I'll haveto check the cam chain,of course, but I think the pump is rotating).

OM617.951 and OM617.952 Lift pump Repair kit W126.120 300SD W123.133 300D
OM617.951 and .952 Lift pump Repair kit W126.120 300SD W123.133 300D

OM601, 602, 603 Lift pump Repair kit
OM601, 602, 603 Lift pump Repair kit

Diesel Lift pump repair kit OM606.962
OM606.962 Diesel Lift pump repair kit

OM617.912 Lift pump Repair kit W123.190 300TD & W123.130 300D
OM617.912 Lift pump Repair kit W123.190 300TD & W123.130 300D

A bit late,but:
Lift pump valve kit: Bosch 2 447 010 011 =MB 000 090 02 10
...or ,the other type-Bosch 2 447 010 044 = MB 001 586 12 07 (which an italian site states are for bio-diesel)-on early om615,616,617,621 engines
http://www.rased.it/productsearch.php3?oem_id=03
... the o-rings are green now insted of black.
You need to verify which valves your lift pump uses...pics here:
http://www.laso.de/pdf/lkw2007/DC%20-%2007%20-%20Injection%20Parts.pdf