Whoever put the brakes on, did they clean the shaft that actuates the brake shoes and put grease on the tip where the brake shoes touch the shaft and on the part of the shaft that runs through the backing plate? Also, did they grease the ends of the brakes shoes where they pivot?
Check your brake cable too, it might not be working freely. You adjust the back brakes buy turning the wing nuts at the back brake lever. One wing nut is for the foot brake and the other wing nut is for the left side hand lever. To loosen the brakes, turn the wing nuts counterclockwise. To tighten the brakes and get a better pedal, turn the wing nuts clockwise.