In my daughters 77 F100, the engine races slightly between shifts. Yesterday, I took the truck for a ride and when I put it in drive, I
stepped on the accelerator and the engine raced before going into gear.
There is also a slight flex/movement in the steering column when shifting occurs. There are no apparent leaks to the transmission or engine for that matter.

 

REplace the modulator on your tranny with a new adjustable one. THat might cure the slippage.

 

I will get give the modulator a shot, much appreciated. Follow up question, is it a good idea to get a trans flush. The truck had 94K and was a daily driver up to about 5yrs ago. I put new front brakes, drag link, tune-up and she's put about 2K since purchace. Jiffy Lube charges $160 for complete trans flush. Fluid doesn't necessarily appear burnt.

 

I'd check the modulator first. Maybe check the vacuum hose to it to begin with. It could of popped off on either end or have a crack in it.

 

I'll give it a try. thank you again

 

Don't trust jiffy lube, they are a bunch of hacks. Even if you know very little about your truck, you know more than they do.

A flush probably won't help. If it doesn't smell burnt and is still red, then you don't need a flush. What's happening is the clutches and bands are worn out. When they used to engage, they engaged right away, but now that they are worn out they require more time to move the fluid out of the way to grab the drum or planetary or whatever it is. This is why a slipping tranny runs hotter, more slippage, more heat.

Another thing that is worn out are all the springs and valves in your valve body. It is now easier for the previous gear to be released before the pressure can build to apply the next gear

If you want firmer shifts, you can try switching to type F, or lightening up springs in your accumulators.

But what you really need is probably a good old fashioned overhaul if the shift flare occurs in all gears.

 

I have in the past had vans that I've bought that didn't drive. I have dropped the pan and replaced the tranny fluid and filter with the manufacture specified fluids (late 80's GMC 700R4) and had success restoring the trans that way. I thought the viscosity of the fluid played a significant role that way. I put quite a few miles on those vans after that.
I liked your explanation of the way in which the trans works you provided, and i'll save that fluid exchange money for the overhaul.