SLIPS IN FORWARD DRIVE RANGES
DRIVE RANGES
1. Fluid Level Low. Add fluid and check for leaks.
2. Fluid Foaming. Check for high oil level, bad pump gasket or seals, dirt between pump halves and loose pump bolts. Replace pump if necessary.
3. Throttle Linkage Mis-adjusted. Adjust linkage.
4. Gearshift Linkage Mis-adjusted. Adjust linkage.
5. Rear Clutch Worn. Inspect and replace as needed.
6. Low Hydraulic Pressure Due to Worn Pump, Incorrect Control Pressure Adjustments, Valve Body Warpage or Malfunction, Sticking, Leaking Seal Rings, Clutch Seals Leaking, Servo Leaks, Clogged Filter or Cooler Lines. Perform hydraulic and air pressure tests to determine cause.
The torque converter if bad, will be bad in all gear selections, not just the forward drive ranges. Have you tested to see if it slips in manual 1st or 2nd? A little bit of dirt and grim won't hurt anything, if it did, the second you ran oil through the filter you'd ruin it as fine particulate exists even in brand new filters. Hell, All the friction material was worn off my front band and I was running black trans fluid full of crap for half a year. Since I was swapping the engine in my truck already, I replaced the front band and it works fine. You can have some debris in the system as the filter will catch most of it. My buddy had a gear shatter into pieces in his transmission and it still drove another 800ish miles before he could afford to have it rebuilt.
My guess, is you forgot a gasket or O-ring when you were rebuilding it. Did you buy a rebuild kit, or just a clutch and band kit? Did you do anything to the valve body when you had it out?