At 120000 miles i was driving the interstate and overdrive kicked out , a few miles later trans temp lite went on. i imediatly pulled over and shut down the truck for 30 minites. i noticed fluid leaking from the front of the trans . after cooling down drove slowly home , while driving i put the trans into neutral when i could to keep fluid moving but not using trans. my shop guy flushed out trans and removed check valve. truck worked fine for about 500 miles no leaks . then while driving 3rd gear went out , all i had was 1st and reverse ! no trans temp lite ! while limping home i would occasionally go into 3rd them i would lose it again !
so i changed the filter and fluid (atf+4) and took it for a test drive . it shifted fine for about 2 miles , then 3rd was gone again ! just 1st and reverse. i checked the cooling lines both were cold ! when i changed the fluid i blow out the lines till no trans fluid came out . i also tried to blow compressed air into the line conection on the trans the top connection would accept air to a point then blow fluid back out . the lower would not take any air , like there was a one way valve ? is the trans toast or should i check something else ? i also have a p1740 code coming up .

thanks for your help !!
Kevin