I rebuilt my C4 from my '67 Mustang, but am having trouble with it shifting. It will initially shift thru the gears, but when I stop, it will not return to 1st gear. It will not downshift when I slow. I can manually shift into 1st, 2nd or 3rd. I pulled the valve body & verified it is built right. The shift kit I installed did have me remove the Accumulator spring. Any suggestions on what the problem is and/or troubleshooting tips?