I apologize for the lengthy post, but I want to make sure to get all the facts down in the hopes someone can help me. My 1976 F-100 has a 302 with a C4 automatic 3 speed transmission. The guy I bought my truck from had the tranny rebuilt and had a shift kit put in it. I bought it maybe 6 months ago, so the tranny has very low miles on it as far as I know. Everything was working great until I decided to replace my stock mechanical fuel pump with a Holley pump/regulator. After installing said pump and setting regulator I got ready for a test drive, put the truck in drive and it just idled. When it finally shifted into gear, after idling for a minute, it would begin to accelerate but when I gave it more throttle it slipped out of gear and began rolling backwards. At high RPM's it will try to accelerate, but just slips out of gear again. I started from the ground and worked my way up, first checking my tranny fluid. It was low, so I added fluid and it started shifting properly again (not as strong as it was prior though) I drove it around for 20 minutes or so and it started the same problem again. I moved down to the tranny and noticed fluid was leaking from somewhere. I thought it was the pan gasket, so I pulled the pan, changed the fluid and filter, put a new gasket on and replaced the modulator (replaced with same color that was in it previous) While having the pan off, I checked for metal shavings, funny colored fluid/burned smelling fluid etc. The fluid/filter and internals looked brand new, very clean. I then checked all my hoses/vacuum lines and replaced some of them including the modulator line. Still had the problem. I noticed, while under the truck, that the shifter linkage was very loose and worn out, so I bought a B&M Z-Gate floor shifter and converted from column to floor. While replacing the shifter I found out where the leak was actually coming from, the speedometer cable. It is leaking where the cable enters the tranny. The floor shifter is done and working great, so I took it for a spin. I put it in drive and same problem. I then manually shifted to 1st gear and was able to accelerate. I continued accelerating and manually shifted to 2nd with no problem. When ready to hit 3rd I shifted up to drive and same problem. Reverse also works fine. I don't know what else to do. I don't know enough about trannys to drop it and try to figure it out. The cable leak should not be effecting the shifting if the fluid is full as far as I know. This is so frustrating because the tranny was working great and shifting really strong before I replaced the fuel pump. Does anyone have any suggestions? I would welcome ANY advice, please.

 

I don't feel replacing the fuel pump would cause the trans to start acting up. It's just one of those freak things that happen from time to time.

You've already replaced the vac modulator, filter, and checked the fluid level. I'm thinking it could be an internal fluid leak, like pump seals, clutch drum seals, sticking spool valves in the valvebody or maybe even a loose valvebody. Might be a good idea to take it to a trans shop and see what they say.