C-4 trans slips out of gear at start up

[74bronk]

I have a 74 with automatic C-4 I’ve noticed last couple time taking it out (while cold and been sitting and backing it out of garage) when I start it up and put in gear reverse will work but I’ve had it when I put in drive it engages then drops off. The trans completely slips out for about 10-15 seconds I’ll shift around then it comes back. Is this something ominous with trans or can it be quickly fixed with an oil change and some service or some additives to oil? Like Lucas oil treatment? Any help appreciated…fyi Vehicle does sit a lot..I do try to run it once a week but the last month been working on other vehicles in garage.

 

[Broncobowsher]

First, check the fluid level. Sounds like you are just low.

If it is low, why? Fluid doesn't just dissapear, it goes somewhere. Either there is a pretty good leak on the ground, or it is staying in the powertrain. There are 2 places it can go. The most common, the vacuum modulator gets a pin hole in it and manifold vacuum sucks it out slowly. With the engine OFF, unplug the vacuum hose at the mudulator. if transmission fluid runs out of the hose (that is why the engine has to be off, or vacuum would suck it into the engine), the modulator is bad. Also pull the fill plug on the transfer case. Massively overfilled? Seal between the transmission and transfer case failed and the transmission is draining into the transfer case. That is a surprise when you pull the fill plug and oil comes out like you pulled the drain plug.