Hi all! First poster here. What a great informative website.
Anyhow, I just bought a '89 f150 with the 300 and a 4spd for 1500. Lariat with working A/C and 145k.
The other owner had a fuel leak that he couldnt locate.
Anyway, after a good tune up and some other preventative Maintenance, I went hunting and found fuel coming out of a vacuum line going to one of the air solenoid valves on the back of the block. it had built up enough pressure to blow the top cap and spring off of it! So I go to Ford and buy a new one for 46 bucks. after installing it-same leak, same spot. After tracing the line back to the fuel pressure regulator, I pulled off the vacuum line from the reg, and started it up and noticed a couple of drops of gas coming out of it. It looks to me that the diaphram has a small hole in it so I ordered one along with a new plenum gasket.
My question is... Has anyone had the same problem?? This thing runs flawlessly. No hesitations or nothing, just the leak! I'm just curious.
Thanks in advance, and take care, Fellas!

 

Diaphrgm failures are fairly common. You diagnosed it correctly (fuel in the vacuum line)and solved it correctly (replacement).