I screwed up! Replaced my radiator the other day and must have lost a lot of transmission fluid. Drove to work just fine then going home third gear stated slipping. Checked the fluid level and was very low so I added Lucas slip stop to top it off but third gear just wont catch. Reverse, first and second are fine and there is no trans fluid in the radiator. I know a diagnosis over the internet is hard but what should I expect to hear from a mechanic? Parts, rebuild, new transmission? thanks

 

Did you top off trans with it idling in park on a level surface? Are you 100% sure that its full? You will get a lot of false readings on the dipstick immediately after adding fluid due to residual tranny fluid in the dipstick tube.
And dont bother with the tranny in a can crap, just add the correct fluid. If it is slipping, nothing in a can is going to replace the missing friction materials on the clutches.