2.5na Diesel lift pump type

Looking to get new lift pump but looks like my 90 has a series 3 lift pump? Is this the case its a 1986 ex military 90. When I search for a pump by engine / year the pump looks different. Can i fit a newer type pump or would it be best to fit a series 3 pump?
The problem I have is its keeps drawing air in and have to bleed the system then its been OK again for weeks sometimes months at a time.
Happened again this morning but the operating the lift pump by hand seemed to struggle to get fuel to the top of the filter and I managed to get home only 3 miles and had to bleed it 3 times along the way. Not sure if the lift pump is the problem but seems a good place to start as the one that's on it is nearly 40 years old.
So far I have thoroughly cleaned out the tank changed the filter and tightened all fittings to no avail.

Quite likely the diaphragm in your lift pump has a pin hole in it.
If your truck has pump with top fitted by screws you can get a service kit.
Modern replacements are pressed together and have to be replaced. Do not expect it to last anything like 40 years.

When I had air getting into the system on mine it was a cracked fitting on the top of the tank. Obviously discovered after changing the lift pump.

They are pretty much the same, Delphi seem to be the better brand these days but even they are hit and miss. I had to make a spacer as the arm was slightly too long

Cheers all
Will try and get replacement diaphragm kit then sounds like the older pumps are better quality than replacements these days.

Just for any future looks at this thread - The lift pump was a series 2a / 3 pump AC Delco could only find a britpart one to replace but has solved the problem. Will see how long it lasts.
Was going to get a service kit for the original pump but the linkages were all well worn.

Lasted just a month with not much use. This time stripped all pipes off from tank to pump and filter and found that the fuel tank pipes had been fitted the wrong way round so the return was going to the lift pump and was full of crap from bottom of tank. The proper pipe to lift pump has a gauze filter in tank to prevent this. I've had it over ten years now and never had these pipes off. Hopefully that's my fuel probs over