I feel as though my F6 this year is unable to maintain prime on the fuel pump (stock mechanical). My F5 on the other hand, will start in an instant (same stock mechanical fuel pump). What can I do to keep it in prime (no, I don't want an electric pump!).

 

If your pump looks like this or at least close you'll note that it may contain a set of check valves so that the fuel won't back wash to the tank. One or both could be compromised as well as the diaphragm itself. Most all mechanical fuel pumps have a diaphragm air vent. If the pumps diaphragm is split or has a pin hole in it you can usually see fuel seeping out the vent. Post a pic of your pump if you can. There are so many types out there that could have been mounted on that truck over the years it's hard to give you advise on what to check first.
Capt'n

 

I think two things cause that. One is a leaky hose that goes from the firewall (end of steel line) to the pump. The other is a steel fuel line full of crud. The crud gets under the check valves (see above) and lets them leak enough to drain back, but not so much they affect normal operation.

When BunnyBlue52 was over at my place, we replaced that hose on his truck, and also rebuilt the carb, and added a fuel filter between the pump and carb, so it was completely empty. I figured we'd have to crank forever to get it running. It cranked less than 5 seconds before firing off. His steel fuel line had been replaced earlier and everything was clean.

 

I'm no expert on your generation of trucks but I do know carbs and fuel pumps. The answer depends on that you mean by loosing prime. Or more to the point how long has the truck been sitting when you have this starting issue. If it's been sitting less then lets say a couple weeks then the fuel pump is of no consequence for fast starting, the fuel that it would be using to start is already in the bowl of the carburetor. The fuel already in the carburetor can not drain back period. Even if you removed the fuel pump entirely the fuel in the carb would still be there and the truck should be able to start and run for a little bit on it. That fuel will eventually evaporate but that could take a couple weeks depending on temps, type of carb etc.

So the question then becomes what kind of carb do you have and where is the fuel going? IDK if your stock, but for example if you have a 2100/4100 series carb on it they are notorious for problems like this. It's a design flaw in the power valve.