In a medium bowl, mix together brown sugar, chili powder, and pepper. Rub this seasoning all over the pork.
Set the electric pressure cooker to the BROWN setting and add a little oil. Once hot, brown the pork on all sides for about 5 minutes. Turn off the cooker and transfer the pork to a plate.
Pour chicken stock and vinegar into the pot, scraping up any browned bits. Stir well, then return the pork to the pot. Add sliced onion and crushed garlic. Lock the lid, set the valve to steam, and cook on low for 8–10 hours.
While the pork cooks, make the coleslaw: in a medium bowl, whisk together mayonnaise, vinegar, brown sugar, and salt. Toss with the coleslaw mix, cover, and refrigerate for 30 minutes.
When the cooking time is done, carefully release any remaining steam and open the lid. Transfer the pork to a large baking sheet.
Turn the cooker to SAUTÉ and simmer the liquid until it’s reduced by half, about 15 minutes, skimming off any fat and seasoning to taste.
Shred the pork into small pieces using two forks or shredder claws.
In a large bowl, combine the shredded pork with about 3 cups of the reduced cooking liquid. Serve on hamburger buns with barbecue sauce and coleslaw.