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Had brake issues over the week end, everything was working fine up until the third heat and the final when the pads began to stick to the rotor, costing me a second a lap, we cleaned the brakes with brake cleaner prior to the final and a decent amount of brake dust came out. On the stand the axle spun freely and the pads had a decent amount of clearance to the rotor, that didn't fix it and the same issue occured in the final. Someone   suggested that the brakes fitted to my kart are junior brakes, implying that they're to small, they work perfect when they don't stick. Could the brake fluid be overheating causing the cylinders to stick and not retract? or is it a simple case of my brake's being to small... I've attached images of the brakes. Keep in mind i race TAG heavy.

Thanks for any help

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I've had really worn pads where the piston doesn't retract back into the cylinder because its cone out too far and slightly cocked over and stuck.
You might have too worn pads.

Hi Nikola,

I'd recommend getting a brake rebuild kit (new orings, seals and return springs), see how you go with that.

James at MKC should be able to help

Check the fluid make sure it looks  clear

Try bleading them as well 

Check fluid level, if it's too high the pistons won't retract fully when fluid is hot due to no expansion room.

Have you checked the pedal free-play? There needs to be some small amount of free motion at the pedal before the master cylinder lever begins to move. This is adjusted by tweaking the rod length (i.e. rod from pedal to MC).

If there is too little or zero free play then there is risk that the MC piston seal may block reservoir hole (the little hole between the reservoir and the cylinder bore). If this happens then pressure cannot escape from the cylinder into the reservoir (nor can fluid get from the reservoir into the cylinder). So, as the brake gets hotter and hotter the fluid expands, and since it cannot escape from the hydraulic circuit the  pressure then gets higher and higher.

This causes the pistons to extend and push against the rotor, i.e. the brake 'auto-applies'. The harder and longer the brake auto applies the hotter it gets, the higher the pressure gets, the harder the brake applies. The extreme end result is terminal pad fade, where the pads give off so much gas that there is no longer any significant friction between pad and rotor. When the brake (fluid) cools, the pressure decreases and the brake pads retract. 

Just because the reservoir hole may not be blocked when the brake is cold, don't assume that this condition still exists when the brake is hot, it may not. I've seen an Arrow brake do what I think you are describing. It was caused by lack of free-play at the pedal. When the brakes had cooled off a bit the problem disappeared, until it became hot again. Once a bit of free-play was introduced the problem disappeared permanently. 

I've had related sort of a problem with my Arrow brake which could maybe have some relevancy to your problem (maybe not), insofar as it involved fluid blockage between the reservoir and the cylinder. The brake was difficult to bleed (almost bloody impossible). On close inspection I discovered that when the piston was all the way forward in the bore (i.e. forward being toward the front of the kart) that the piston seal was mostly blocking the reservoir hole.

This appeared to be caused by the piston seeming to be ever so slighly the wrong length (too long between seal lip and forward face of the piston where it contacs the snap ring). This was causing the reservoir hole to be mostly obstructed by the piston seal. Some fluid could pass between the reservoir and the cylinder through the constricted hole, but not nearly enough / quickly enough. This issue was fixed by machining about 0.5mm off the front of the piston (where the forward piston face contacts the snap ring at the front of the cylinder bore). This allowed the piston to move ever so slightly more toward the front of the cylinder, fully uncovering the reservoir hole and permitting free passage of fluid when the pedal was released. 

Regards,

John.

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