Fuel pump question. Please help!!!

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My 1961 Cadillac 390 was running great until the fuel pump went out. I replaced it with 3 new or rebuilt pumps and they won't pump gas to the carburetor. I temporarily installed an electric fuel pump and it runs like it did before the mechanical pump quit working. I'm at a loss for what to do next. The car is stock and original and I don't really want to use an electric pump permanently.
Thanks in advance.

[Lexi]

After 3 pumps tried it seems like your problem may be associated with what drives your mechanical fuel pump; i.e. the pump's push rod or the engine component that drives the rod, (cam lobe I believe). The shop manual should provide specific permissible length measurements for the push rod. As the distance of travel is very slight, it does not take much wear to effect the fuel pump operation. Perhaps worth a check, especially if you recently had any extremely high rpm cold starts lately.  Clay/Lexi

[55 cadi]

Try checking the rod that moves the mechanical fuel pump, that would be my first go to.

[The Tassie Devil(le)]

I can't see these rods as a problem because they are hardened steel, and very little chance of wearing out.

The only problem I could see is if the actuating arm on the replacement pump is the wrong shape, or the eccentric on the end of the cam (Held on by a bolt) could be worn away.

But, if the pump was working satisfactorily before it went bung, and the replacement pumps arms are the same, then I would be checking the pump internally to see if the one-way valve/s have come loose.

Bruce.

[fishnjim]

you say the "pump went out", does that mean the diaphragm broke or some other malfunction?   Or did it just stop getting gas to the carb all of the sudden and you thought it was the fuel pump?   Usually after 2 replacements it's not the part that at fault.   
If you put an electric pump at the tank and it's "fine", then it could be something in the line blocking flow and creating a restriction on the suction side.   Flush out the fuel line and blow it with air to make sure it's not stopped up/restricted.(or replace)   They tend to rust over time and that closes off the inside diameter and increases pressure drop.   A piece of rust and/or rubber, fuel sludge, etc. can lodge in a restriction but the electric pump is not suction limited and pressuring the flow in the line not sucking it a long distance.   My Packard fuel line was completely stopped up when I got it.   I had to soak in acetone to dissolve the fuel sludge.   You can also bypass the tank/line and hook up a portable fuel bladder to the mechanical inlet and see if that obviates the problem.   Just be careful with handling fuel in portable situations.  Obey grounding, etc.   Have fire extinguisher present. 
There are test tools for this but a pressure gauge is usually indicative that it's putting out properly.   I keep an inline fuel gauge on mechanical systems for that reason, easy to diagnose.
Mechanicals don't do well with restricted suctions.   I see this alot with people having fuel starving issues on older cars with original parts.   
It's a small task to check the push rod but unless it broke or something, wear is not a sudden failure.   Fuel systems need maintenance like all the other driveline components.   

[Lexi]

As Fishinjim indicated, there is not enough info in your post. Have a look at my several postings on fuel pump problems under carguyblack's post called "1956 Cadillac losing prime to the carb" under the Technical/Authenticity index tab of this CLC Forum, for more information. I presented detailed troubleshooting info there, which should assist. Although for a '56 Cad, most of the info would still apply here. The rods are not usually a problem as stated by another member, but these are antique cars with usually multiple previous owners. We don't know how they may have been treated. Perhaps the rod/cam had 'spun' from previous abuse from another owner which now has presented itself. OR you got the wrong pump lever contact as suggested by The Tassie? If all 3 fuel pumps (rebuilt/new) were handled by the same business maybe they did something wrong. But first, look at my posts and conduct the tests noted, that way at least some concerns will then be addressed/ruled out, thus pointing you in the right direction. Clay/Lexi