Are you filling a whole set of carts to flush your printer? All refillable carts (no matter what ink/fluid is used) for the 3800/3880 printer MUST have Epson chips placed under the reset chips, as per the instructions.
If you are flushing color ink from your 3800 printer, then you will want to fill the carts at least 1/2 way with PiezoFlush, then preform 3-4 Powder Clean cycles, or one Initial Fill cycle.
Please let me know if you have questions or there's anything further I can help you with.
Best regards~ Dana