This was an issue we had back in 2014, but thought it was worth sharing. We had been hampered by brake problems for the last 4 races.
The brakes have either been getting spongy or jamming on, actually sometimes both! I had been bleeding the brakes every race (every time he went out on track!) The slowest mechanic in the west (that's me) has been working hard to fix the issues. Basically just completed another strip of the brakes and new seals all around again, but this time new pistons in the calliper as well.
If you are not on a budget you would just replace the whole lot. But that was not an option so we are trying to find the issue. Changing the Pistons has certainly helped, but bleeding the systems seems to still be required once every weekend. I don't see other OTK kart owners having this issue.
The jamming problem was partly the pistons jamming in the calliper I think due to wear and partly the pads jamming in the calliper slider. The area that the pads sit in is heavily worn.
This kart was supposed to lightly used, but the brakes are basically worn out. The callipers are made from a Magnesium alloy like a lot of OTK parts, which is very soft. It is certainly an area to inspect when buying a second hand kart, pop the brakes pads out and see if there is a divot worn in the calliper where the pad sits. If so it is possible that the pad can start to catch this and start to jam. This system basically has a small sweet spot where the pads work and the pistons don't jam. Brand new pads jam and so do half worn ones, so its juggling act to keep pads in the right condition to keep this kart going!! I actually filed the slider area flat again and this helped a bit more. Would it be cheaper to replace the calliper!! Certainly less hassle!