From the description it sounds as if you knocked the hose off the modulator valve. Make sure it is getting engine vacuum.

Also, you will be doing a MUCH better job of transmission service if you will drain the torque converter as well as the pan. It only takes just a little longer to do this.

Hope this helps.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

No vacuum modulator or TC drain plug on his transmission.

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Timc, did you check and set the fluid level after you did the work? Did you drop the pan and clean it out and replace the filter? More details are always helpful. If all you did was pull the plug and then add some back, I'd drop the pan, clean it, replace the filter, fit a new gasket, add a bit less than the fluid you drained, then start it, set the 25C level with the dipstick tool, then run it until the fluid is 80C and set the level. Most seem to target the middle of the 80C range for the final level.

Good luck.