Jeff's 727 rebuild thread

Ok Jeff. Let me try to understandably explain what I think is going on. When you select D at a stand still the only component that is hydraulically activated is the forward clutch. The clutch frictions are engaged in the front planetary ring gear & turn this ring gear clockwise. The power flow through the geartrain in 1st or low gear uses both planataries for the gear reduction of 2.45:1. Due to the geartrain rotation the rear planet carrier wants to turn anti clockwise. This is prevented by the 1 way roller clutch (Sprag) holding the rear (low/reverse) drum against counter clockwise rotation. The rear planet carrier is splined into the low/rev drum so the drum is also held against counter clockwise rotation. This causes the rear ring gear or annulus, which is splined to the output shaft to rotate clockwise at a reduced speed giving you 1st gear in D or "Breakaway 1st to use the Chrysler term. With me so far? Good!
Right, Now you shift into manual low or 1st gear. The forward clutch is engaged but now the rear (low/reverse) servo is hydraulically activated clamping the rear band around the low/reverse drum preventing counter clockwise rotation. This will give you 1st gear as before & also the braking effect from the engine on decelleration. On the shift from 1st to 2nd gear, the intermediate (front) servo is hydraulically activated, clamping the band around the front (direct/reverse) clutch drum preventing rotation. As soon as this band is applied, the sprag releases, because the low/reverse drum now rotates in a clockwise direction.
Still with me? Now if there was a fault in the valve body that was causing the front servo to apply when you select D this would give you 2nd gear at take off. This is where I am certain your issue lies. You will have to pull the valve body & check the 1-2 shift valve to see if it is hanging up & find out if you have any cross leakage that is causing the intermediate servo circuit to be pressurised.
I hope I didn't lose you in the explaination & I will keep my fingers crossed for you. You can sort this!!
Cheers. Gaz