Your caliper pistons and seals are most likely full of corrosion and build up.
look at the pistons closely before any amount of brake pedal is applied, look at the distance the caliper pistons are at.
Drive the kart and get it to act up, then look at the distance and see how much further the pistons are sticking out
I almost wanna put money down that your calipers and master cylinders are in need of rebuilding, or a good honing and cleaning.
Try duplicating the problem, as I suggested first, paying close attention to the calipers first, if you confirm that the pistons are not returning, then take the calipers apart, and master cylinder, and rebuild them, flush and bleed them, and good to go.