Looking for ideas/input on a brake situation I have. I run a 2021 Birel type chassis with the Freeline 132x2 caliper. This design has 2 o-rings per side one of which is a square type o-ring. It was my understanding that this is referred to as a deflection type of o-ring that basically deforms when the piston moves out and when pressure is released causes the o-ring to return to its original shape which in turn pulls the piston back into the caliper. This also creates a self-adjust effect as obviously, the o-ring can only deflect so much. However, I have a situation where one piston basically returns all the way back into the caliper which in turn creates long pedal travel. The other piston works as it should. I have bled the brakes several times with no improvement. Any ideas? Do I have the o-ring theory wrong?
Have you pulled the pistons out of the caliper to examine the o-rings/square-rings? It may just be time for a rebuild. You should also check the bore and make sure there is no debris or scoring that could be catching one of the rings forcing it to tumble like a spring and return the piston back to its starting point.
Quick and dirty “fix” is to shim the brakepads so the piston movement is reduced. Then - if piston returns for reason or another, no big deal as it can only return a small amount. Fix the brakes later 
Have you modded brakepedal/master cylinder linkages or stops? Can the mastercylinder piston go all in? I had something similar long time ago, the master cylinder couldnt cycle the fluid and took it from another piston
Your are right about o rings how they work. I remember some brake manufacture article about square o rings - An area of constant research and development that is hard to master
Some brake fluids have fancy name but are super DRY. Have noticed this while changing from fluid brand to brand. Friction/stiction from the seals dissappeared. From not retracting to buttery smooth
Greg,
Over the winter I totally cleaned the brake system, but the o-rings looked good so I reused them. It was functioning as expected up till recently. But its only got about 2 hours of use. Curious…when you do a rebuild do you replace pistons too?
Mikko - the brake fluid is an interesting point. I use DOT4 type brake fluid which is supposed to be hygroscopic. Which type did you find to be “buttery”?
I didn’t. I just removed them, cleaned the entire caliper bores and pistons, checked for any imperfections on their surfaces and replaced the seals. did the same on the master while I was at it. No real issues. In the four or five years I have run my second had kart, I have only replaced the o-rings once. I have flushed the fluid multiple times per calendar year.
It is compared to DOT3 or DOT5, but what does Birel dictate for use in their system. You cannot mix or swap fluids. Swapping may require different seals.
It could just be something simple, like a little crud on the bore of the caliper. If you have about an hour, you can pull the pistons and check. Also, not being as familiar with Birel Master Cylinders…are they single or dual piston? On the OTK, they are dual meaning one piston per piston on the caliper. I have seen some that use a single larger bore one the master that Y at the caliper to feed both simultaneously.
Have you checked the master cylinder? I’m not familiar with the freeline system but for others that I’ve used it’s largely the master cylinder that determines how far the calliper piston retract. Firstly I would check the reservoir breather is not blocked. A quick check for this would be to remove the cap and pump the brake. Does the problem persist. Failing this I would strip and thoroughly clean the master cylinder including the small passages between the cylinder and reservoir. Hope this helps.