OTK brake help needed pedal goes all the way down after 2-3 applications

I am having a major issue with my brakes, my brake pedal will go all the way down to the crash bar after about 2-3 applications on track only. I can’t replicate this issue on the stand at all. What seems to be happening is the caliper pistons are retracting as i am driving.

To try and solve this problem i have replaced all 4 seals in the master cylinders, and the seals in the caliper. I have tried bleeding the brake like you would on a car, with this method
“First make sure that both master cylinder pistons are fully returned as far as the retaining circlips.
Fit the bleed tower but leave the tap on the tower closed.
Fill the bottle with dot 5 fluid.
Remove the reservoir cap on the cylinder you are connected to.
use a rag to soak out all of the fluid in the reservoir
Now open the tap and allow the reservoir to self fill until 1/3 full only then close the tap.
(if the reservoir does not fill when the tower tap is open the piston is not fully returned and the cylinder will need cleaning and checking)
Now re fit the cap & seal with the reservoir only 1/3 full.
Now open the bleed nipple on the caliper.
Next open the tower cap and allow the fluid to flow until clear of bubbles then close bleed nipple.
Close the tower tap and remove.
Re fit the cylinder plug ensuring the rubber seal is in good condition.
Repeat the the process for the other cylinder and you are done.
Just check that when your brake is set up the two push rods on the balance bar are totally free of the master cylinder pistons. any pressure from these rods on the pistons will cause brakes to drag when hot.”
as well as the method in this video

Regarding the caliper seals. they have a small bevel on the outside diameter and i did not take note of witch way they were facing when i removed the old ones. does any one know witch way they face?
I am quite certain no air is in the system the pedal feels grate when i check it on the stand.

Can’t help with the caliper seals without taking mine apart. The seals in the MC look as if they positioned correctly. I might have a spare piston with seals on, but it’s buried deep inside the trailer right now. Will look later.
Is there any chance that one or both of your pads are dragging on the rotor? Maybe a loose axle. Have you tried tie-wrapping the brake pedal to the bumper, walking away for awhile, come back and check for a hard pedal? You said you were using Dot 5 fluid. That should be 5.1.

Does the caliper have any damage from an axle moving at all previously? Generally this burrs up the caliper when the disc contacts it and the pad wont slide in and out freely. Also check that the magnets on the pistons still have good magnetism.
You seem to have done everything correctly bleeding wise and only filling the reservoir 1/3 to 1/2 after bleeding. From memory the caliper seals don’t have a particular orientation but could be wrong here.
When the pedal goes to crash bar are the pads on the disc or retracted back to their resting position?

no issues with dragging. I will check the axle. I haven’t tie-wrapped the brake pedal for along time, but what i have done is sit and hold it on for about a min I didn’t feel any drop in the pedal. A bit of a typo I am using 5.1.

the caliper does have some damage. I don’t feel any major burs, but i will does some light filing to be safe. The magnets seem to be good.

This looks to be the older Otk brake rotor from a few years ago. Unlike the current brake system that used DOT 5.1 brake fluid, the older system uses DOT 5 brake fluid, usually purple in color. This might be your problem.

On failure, are both MC pistons in the same position or is one pushed in farther than the other. Looking to see if half the system can be eliminated from the problem.

Most of the time the inboard master is compressed further, but not always sometimes they are equal.

Does the brake come back to normal if you pump the pedal or is it a matter of ’ when it’s gone it’s gone’?

That’s kind of good news. Both pistons should always travel the same distance in. You either have a mechanical failure or air still in the line. Start with trouble shooting the inboard MC, the outside half of the rear caliper and the brake line. To isolate the problem further, you could swap the brake lines at the caliper. Less to blead that way. When you pulled the MC and caliper apart, were there any signs of corrosion in the bores or scratched pistons? I’ve had a braided brake line damaged but have never seen one fail.

Yes if I pump the brakes the pedel comes back.

The bores and pistons in the caliper look good. The MC pistons look good. The MC bores only had minor corrosion in the snap ring grooves.

Does this damaged reservoir diagram concern anyone?

I’ve had OTK brakes do the same thing when the axle set screws come loose and allow the axle to shift. It’ll center when going down a straight, and then shift to one side in a corner and not have brakes for the next corner.

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Thanks i will make sure to check for that.

This sounds typical of pad knock-back. Which happens when something causes the rotor to push against the pads and thereby pushing the pads further into the caliper which means you have to pump the pedal to reset the pads against the rotor before you get any brake feel.

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problem solved, the axle was loose

The set screws were just tight enough that you couldn’t tell with the kart on the stand lol.
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As a follow up, using grub screws to locate the axle is imo a very poor way of doing things.The grub screws are provided by the bearing insert manufacturer to stop the inner bearing race spinning on the shaft/axle not to resist the axial loadings of ‘high g’ cornering.
Better methods include fitting clamp on 2 piece split collars next to the bearings or fitted tubular spacers between bearing and sprocket carrier or other clamped on part eg. hub.

Caliper seals on self adjusting (as opposed to spring return )brake calipers, as well as performing the obvious function, retract the piston so that when the pedal is released the pad should be pulled clear (0.5mm?) of the disc/ rotor by the piston magnet.
Effective retraction of the piston is dependent on the design and material of the seal and the shape and dimensions of the seal groove in the caliper.
Alexs’ pics above show the seal shape and the ‘bottom’ of the groove will? be machined to accommodate.
Apparently the larger outside diameter end of the seal should face the disc but I doubt if fitting it wrong way round will be a safety issue but may affect the retraction.

The detailed method of bleeding OTK brakes described above starting" First make sure…was posted on Karting I. co.uk some years ago by ‘Race instructor’ much respected in UK. “Any other method will lead to trouble.” he claimed…