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So my '66 (Bonneville) with the TH400 slips a bit if you give it throttle when shifting between 1-2 after around 10 minutes of driving. Of course I don't wanna take it in for tranny service/rebuild until it actually becomes a problem, so I bought a 24 fl Oz bottle of this stop slip stuff. A friend swears that for a year it really helped his worn transmission, so for $11 I figured I'd give it a shot. The only thing is, I don't want to risk overfilling the fluid.

I haven't touched the transmission fluid since I've owned the car for the past 2 years because I've only put ~2k miles on the car and the original owner said he "had transmission work done" a year before I bought the car.
Can I just dump all 24 oz in without worrying about overfilling? Anyone had any experiences with these Lucas "quick fix" additives? My slipping is minor so I'm trying to avoiding major repairs for now, being a broke college student and all.
 

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There is not a miracle cure in a bottle. Never has been never will be. Will it take the 24 oz without draing some.. probably. Worth the $11? I've spent more on less...
 

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With one bolt, you can pull the vacuum modulator from the side of the engine and drain about a pint or so of fluid without messing with the trans pan. Quick and easy. Then add your snake oil. In the '80's, I had a '64 Ford Galaxie with an FMX tranny that wouldn't move when you put it into gear. I added a can of KW Trans-X, and the tranny worked perfectly for another 9 years until I sold the car. If your TH400 slips on the 1-2 shift, it's probably a pressure problem and not a clutch problem. The snake oil may work...worth a try. When you're in there, you can take a small screwdriver, and turn the screw in the modulator(where the vacuum hose connects, inside the nipple) clock wise a turn or two. This will cause the trans to upshift into 2nd gear later, with more line pressure. That too may help or cure your problem. It costs nothing to try.
 
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