C6 won't engage after sitting for 5 years

[DevanTheDude]

Hey everyone,
I've got a few questions about my newly-acquired 67 F100. It's got a C6 mated to a Mercury 360, 2 wheel drive, 2-barrel. The truck is pretty rusty, but that will be taken care of eventually. My biggest concern is getting it running right. It sat in a field for 5 years before i bought it. So far, I've been able to get it running and stopping, but the trans has become an issue.

The C6 will not engage into any gear unless I give it revs. Once in gear, it wants to fall out. I checked the fluid and its at a good level, just below the fill line when warm. I went ahead and dropped the pan. Fluid looked decent. No metal shavings that I could see. Filter wasn't clogged, but it had some dirt and grime in it.

Are there any things I can easily check before pulling the trans out? Also, along those lines is it easier to just pull the engine/trans out as one piece? I've got a cherry picker and leveler.

Thanks in advance!

[carnuck]

Some purists will frown at this, but the hard seals in the trans from sitting need some extra sealing to fully engage. Dexron III (or higher) and not Type F fluid? I added a can of STP engine oil treatment to mine years ago to regain reverse (Lucas transmission treatment is the newer stuff). Let it sit in gear for a bit to keep it engaging the clutches or they will fry shortly. After doing this and waiting a good minute or so to engage, then give the gas a little.
Re-check fluid level while running (I forgot to mention, It's easiest to pour directly in the trans pan before putting it back up) Make sure the trans cooler in the rad isn't clogged. It's a bigger issue with other brands, but I know people who lost brand new transmissions to a clogged cooler in the rad.

There is a can of stuff called Kooler Kleaner or similar that screws onto the fitting at the trans and flushes it out.

[cep62]

X2 on Lucas

[DevanTheDude]

Thanks guys, I'll give it a try. Nothing to lose at this point.

[DevanTheDude]

Update: I ended up dropping the valve body to clean and inspect. Reinstalled everything with a new filter, 5 quarts of type F and a full bottle of Lucas automatic trans treatment. Started it up and let it idle. Added another half quart to get it up to the full line.
Jumped in the truck, shifted to reverse. It engaged into reverse! Holy ****! Put it into drive. It engaged! Just got back from driving it around the block. It totally worked