Massey Ferguson MF 290 --- Mechanical Lift Pump

Hello!
I can't get my tractor to start after changing fuel filers and replacing the mechanical lift pump. I accidentally damaged the threads on the part of the lift pump where the in/outlet are so I recovered that part from the old one, cleaned everything and put it back together.

As I push the lever on the lift pump, it pumps away, but after about 20 squeezes it will gradually loose all compression. Is that a definite sign that the lift pump has a problem? Or is that perhaps how it's operating? I know that depending on the cam's possition in the crankcase sometimes the lever does not get good contact, but I have tried to turn the engine over, and the problem is the same everywhere.

If I wait for a a while, the compression comes back but goes away again after I pump a few times again. I suspect it draws in air? Any experience with this?

When quits pumping the system is pressurized to system pressure of 3-5 psi or what it operates at to supply inj pump.

(quoted from post at 13:03:11 02/20/22) When quits pumping the system is pressurized to system pressure of 3-5 psi or what it operates at to supply inj pump.

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Thank you for that info. I will then have to check the high pressure diesel pump. I did give that cav pump a full clean and resealing a few months ago, and it started up nicely afterwards , but then it sat for maybe 2 months before I got the time to try again.

I get diesel from the lower and upper vent screw on the pump when priming the lift pump, but when turning the engine with the starter to bleed the air out of the injectors, I only see about 1 or 2 drips of diesel per second.
As I recall from earlier, the pressure from the upper vent screw and also from the injectors should be higher? Maybe I will check the plunger in the endplate. Maybe it's stuck in a position that doesn't allow enough diesel through. I thought it could not be the stop pin because I just cleaned and resealed the pump a few months ago, and left the tractor with the stop knob pushed all the way in. I was thinking maybe the little spring under the plunger maybe it's broken. I will check! I was almost sure the loss of compression in the lift pump was a sign of a problem, but what you just said makes sence. Thank you!

What brand fuel filters did you use? If NAPA/WIX 3166/33166 did you make the common mistake of placing an O ring in the filter top outer groove? That's the fuel inlet
passage on those filters and must not be blocked.

(quoted from post at 16:08:31 02/20/22) What brand fuel filters did you use? If NAPA/WIX 3166/33166 did you make the common mistake of placing an O ring in the filter top outer groove? That's the fuel inlet
passage on those filters and must not be blocked.
<img src=https://www.yesterdaystractors.com/cvphotos/cvphoto118073.jpg>

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Thank you for that info! I have a filter that looks like the cav 296 type on the left in your photo. So I don't think that is the problem, but I appreciate your input.

Shawn is correct. Loosen the line going into the first filter and see if you get fuel flow. You may have incorrectly installed the fuel filters blocking flow. But that's how diaphragm pumps work. There is a spring on one side of the diaphragm and when pressure in the output section is greater then the spring they stop pumping.

(quoted from post at 06:12:36 02/21/22) Shawn is correct. Loosen the line going into the first filter and see if you get fuel flow. You may have incorrectly installed the fuel filters blocking flow. But that's how diaphragm pumps work. There is a spring on one side of the diaphragm and when pressure in the output section is greater then the spring they stop pumping.

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Thank you! Yes that seems logic now. I only have one filter, it's the long type! One more thing that I didn't mention, was that when I cleaned and resealed the high pressure diesel pump, I discovered something strange. After soaking many of the parts in fresh diesel to clean them I noticed a hole straight through on the end-plate(where the diesel line comes in to the pump) there are two positions for a small pin there, and I suspect the previous owner have perhaps gotten that endplate from another pump and changed the position of that pin, maybe knocking it in with two much force? Maybe it had weakened the metall or maybe he had fixed the hole with JB weld or something?Anyways it was not leaking there before. I just saw that hole first time after cleaning it.
So I soldered that hole as a temporarily fix. After remounting everything the tractor started up and ran nicely. However after about 20 minutes I saw that my solder had cracked and fine diesel mist was spraying out from there.

That's why the tractor sat for maybe two months before I got the new endplate from china and mounted that on. I now wonder if that leak could have caused pressure imbalance in the pump? Maybe breaking some of the spirngs in there, or pushing the plunger so far up that it would get stuck or something.
I will have to open that up and inspect.