Place your corned beef brisket, fat side up, into the Instant Pot liner. If using the Instant Pot rack, place your corned beef brisket onto the rack and then lower the rack into your Instant Pot liner.
Sprinkle the seasoning packet (that came with corned beef brisket) on top of the meat.
Add bay leaves and peppercorns into the Instant Pot liner.
Fill Instant Pot liner with water to cover the corned beef brisket but do not fill the liner past the 2/3 mark (liner is marked PC Max 2/3). It is okay if the top of the corned beef brisket is not covered with water.
Put the lid on your Instant Pot, lock the lid into place, turn the vent to "sealing". On the panel of the Instant Pot, press the Instant Pot "Pressure Cook", "Manual" , or "Meat/Stew" button, set the temperature to HIGH, and set the cook time for 90 minutes.
After 90 minutes, do a "quick" release of the steam pressure. Be careful when doing this as the steam will release quite forcefully. (See the video.)
Once the pressure has returned to normal, remove the Instant Pot lid. Lift the rack with the cooked corned beef brisket and transfer it to a cutting board to rest. Tent the cooked meat with aluminum foil to keep it warm. (If not using the rack, remove corned beef brisket carefully using two spatulas.)
Carefully add carrots, potatoes, and cabbage to the Instant Pot liner. Be careful as you do this because the liquid in the Instant Pot will be very hot.
Once all the vegetables are in the Instant Pot liner, replace the lid on to the Instant Pot and set the vent to "sealing". On the panel of the Instant Pot, press the Instant Pot "Pressure Cook" or "Manual" button, then set the temperature to HIGH, and set the cook time for 5 minutes.
After 5 minutes, do a "quick" release of the steam pressure. Be careful when doing this as the steam will release quite forcefully. (See the video.)
Slice the corned beef brisket and transfer the slices to a large serving platter. Remove the carrots, potatoes, and cabbage from the Instant Pot liner, and add your cooked vegetables to the serving platter along with the corned beef brisket. Enjoy!