I recommend flushing all the fluid out of the transmission.
The best way to do a transmission flush yourself is to:
1. Remove the tranny pan drain plug, and allow fluid to drain from pan (approx 3-4 quarts should drain out).
Note: Steps 2 and 3 are not necessary, but I highly recommend doing them.
2. Consider removing the pan while it's drained to clean the filter screen and the magnets at the bottom of the pan. (This will add an hour or two to the job.) Careful if you remove the filter. More fluid will fall out of it when you loosen the bolts that hold it in.
3. Re-install tranny pan (I use a gasket maker called "the right stuff" made by permatex. It works well, and you don't have to wait a long time for it to cure before you can re-fill your transmission after you put the pan back on.)
4. Re-install tranny pan drain plug, and refill tranny pan (through dipstick tube) with ATF. Put in the same amount that you drained from the pan in steps 1 and 2. Use the dipstick to verify that you have the correct level in the pan (proper method of checking fluid level is with truck running, after having slowly cycled through all the gears from "P" to "L" and back up to "P". Unless the truck is warmed up - which it won't be after doing steps 1 to 4, use the "cold" line on the dipstick.)
5. After having drained and refilled the tranny pan , you can then flush the rest of the fluid out by disconnecting the ATF cooling line coming from the stock tranny cooler in the radiator, then, start the truck and let the tranny pump spit out approximately 1 quart of fluid.
6. Shut off the truck, and top up the tranny fluid through the dipstick tube. Repeat this 11-12 times or so (depending on the capacity of your transmission), to flush the majority of your fluid, with very little waste.
7. Reconnect ATF cooling line to tranny cooler.
8. Run truck for a few minutes, then check fluid level. Top up if necessary. Do a final check of fluid level after you get the tranny fluid up to normal operating temperature.
