OTK brake problems, not working in high speed

Hello I’m new to an OTK just got my first this year and this weekend I was having some trouble with the brake system and wanted some input. I drove the kart for a race a few weeks before this weekend no issues, I come into this weekend the brakes on the stand were firm, they seemed fine, I go out first session the brakes were working at first, mainly in low speed corners but whenever I’d go into a corner at high speed the pedal would just go soft and fall to the floor and nothing would happen (had to spin myself out on purpose to not hit the barrier). After asking around the theory was that there was air in the lines, because in high speed corners there would be some much heat being built up making the brakes soft. We ended up putting new fluid in had a friend help us bleed the brakes to get all the air out and I still had the same problem on track. Someone helped us notice my brake pads were wearing unevenly probably because the axle could have possibly been shifting around not being tight enough. He also pointed out I was also using aftermarket brake pads which could have been causing the problem. Next practice day I’m gonna go make sure the axle is tight, throw some actual otk brake pads in and see what happens.

I wanted to know if anyone had any ideas as to what could be happening to cause this issues? And am i going in the right direction with what I should be doing? Anything would be appreciated

Update: in case anyone encounters the same problem, it was the axle that was moving around causing the braking problems, set screws werejt enough to keep it tight so i put lock collars on the axle to prevent it from moving and the problems went away.

Also sidenote make sure there is no air in your brake system and make sure all the connections are tight but not overtightened.

Your plan sounds good. After the axle is secured in place and the pads have been replaced, clean off the rotor with brake cleaner and center it between the pads.
Tip: Outboard of each rear axle bearing, wrap the axle with a couple of turns of electrical tape. If the tape is disturbed after a session, you’ll know if the axle has shifted.

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Is the axle moving left to right at high speed and knocking the pads back? Does applying the brakes twice cause them to work?

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Definitely sounds like the axle is shifting left/right and pushing the pads back. Make sure you’ve got all those grub screws tight.