The ink in this printer ink cartridge is retained because the cartridge has an internal sponge. This printer ink cartridge is physically identical with the #27 C8727AN cartridge. HP puts less ink into the other printer ink cartridge when new.
HP 56 REFILL INSTRUCTION
Step 1) Put the printer ink cartridge on some paper towels in an area where you can tolerate a spill.
Step 2) Remove the label on the top of the printer ink cartridge. You will find 5 holes.
Step 3) Put the holes on top of the printer ink cartridge. Turn the cartridge so that the single hole is farthest away from you and the other 4 holes are closest to you. You may fill through any of the following holes:
- The single hole farthest from you.
- Either of the two holes closest to you.
Step 4) Keeping the printer ink cartridge on paper towels, take a syringe bottle and fill it 1/2 full of ink — about 15 ml. When new, the cartridge holds 19 ml. When full, the printer ink cartridge holds 19 ml.
Step 5) Insert the needle fully into the hole. Get the tip of the needle to the bottom of the cartridge. Inject the ink very S-L-O-W-L-Y and watch until the sponge is saturated. You must get the needle deep into the hard sponge.
Step 6) It is not necessary to reseal the holes.
Step 7) Set above a paper towel for an hour as a drip precaution, perhaps over a small cup. Expect a few drips. Do not allow the printer ink cartridge to touch the paper towel at this point, since the towel will draw out the printer ink.
Step 8) For best results, let sit overnight before using. Tape over the vent holes if you plan to store for awhile to prevent drying out of the ink.
Source: oddparts.com