(UPDATED) Braking problems, can't lock axle

Can anyone tell me what’s wrong with my brakes? OTK SA2 system on a Kosmic kart. Can’t lock the axle with my hand on the pedal with the engine running. Can only lock the axle on circuit in low speed corners with a lot of effort. Plenty of pressure in the pedal, no air in the lines, bleeds straight through perfectly fine. New seals in the caliper as well. A few things of note are the pistons in the caliper look pretty worn, fluid does appear to come up a bit dirty after a while, disc is aftermarket but had no problems with it up until this point. Using Dot 5.1 fluid.

From what I can see it doesn’t look like the pads are squeezing the disc enough in order to create any worth while braking despite all of the pressure in the pedal? Seals are also a different shape to what I had in the caliper earlier but have had no leaks or retraction problems.

Would appreciate any input at all

Okay. Got back to working on the kart only to discover I’d lost pressure in one line. I opened up the caliper after some bleeding (which yielded no results) only to find one of the seals had perished. Bits of seal all throughout the piston chamber.

Having had enough, I decided I would just rebuild the whole system to leave no margin for error.

(For reference, I’ve been having braking problems a lot longer than this post has been up.)

Rebuilt the master cylinder and caliper completely, managed to get pressure back and regained enough braking to take my kart to the circuit!
I ended my first stint after 10 minutes only to find fluid had leaked through the caliper seals all over my pads and rotor.

Seals I’m using are suitable for ‘‘BS6’’ systems. I can’t find any information on what connotates a BS6 system over a BSD and SA-2 system but other websites selling the same seals assure they are fine for all three.

In which case am I using the wrong fluid? I ask this because I know that DOT 5.1 is standard for BSD systems, and by that logic, the BS6 + BSD + SA-2 seals I’m finding should all work fine with that fluid?

I apologise for my lack of knowledge in advance as well, still relatively new to the world of owner driver karting.

Have you done a proper bleeding ? I mean otk systems are a little odd when it comes to bleeding and they have a little bit diffirent way off doing it . Also you need a bleeading tower to do it right . Its not the same as we do it in a simple car or motorcycle brake system .

Here it is the whole thing in a video ,
Hope it helps

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The way I bled them was by draining the system by hand opening and closing the bleed nipple whilst pumping the pedal, then filling the lines with the tower and then getting fluid into the caliper by bleeding by hand again. All whilst making sure there was enough fluid in the cups so that no air entered upon pumping the pedal. Fluid ran solid with no air bubbles at all, pushrods ran completely straight with a perfect amount of pressure.

Edit: Wording

Check for a missing key in the brake rotor.
Wipe the rotor with brake cleaner and install new pads.

Never checked for a missing key. Tried 3 different sets of brakes pads ranging from almost dead to brand new and it made almost no difference. Never got round to wiping the rotor down with brake cleaner but I can at least tell you that it was very clean upon visual and physical inspection.

Was it good for you before and then changed? Might be worth looking at the pivot point that’s chosen at the pedal and\or the master cylinder. Although I don’t think the MC has any leverage options on the OTK systems?

If there’s a hole in the pedal available for the brake rod that’s (/edit closer to pivot bolt on chassis. ie further down), worth trying if everything else checks out hydraulically and mechanically.

Upgraded to a newer second hand brake system with such problems. There are multiple holes on the pedal for the brake fork to slot onto if that’s what you mean. Think I have the most optimal one anyway. Messed about with pedal tightness and removed all of the slack so any pressure put onto the pedal triggers the brakes.

Okay, thank you, I’ll add this to the list of things to try.

This may be a dumb place to look, how about the pedal stop? I had a problem with the pedal and it took me forever to figure it out.

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Changed the pedal stop bolt as well to get the tension right, didn’t do much.

it seems that everything should be working, and you mentioned that there WAS some difference when changing pads. if there is room, have you tried shimming the pads, to get the contact point even closer to the rotor?

i’ve had your symptoms before, where everything seemed like it should work, and it just turned out to be my pads were worn, and the caliper piston was at the end of its travel.

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Newest pads I tried were practically brand new in terms of wear. My current theory is that the pads I’ve been using are all caked in old brake fluid and other kinds of gunk as they were from an older brake system I was doing a bunch of maintenance on, and a lot of this has transferred onto the brake rotor.

A bit of an obvious oversight I realise so you’ll have to forgive me on that one. I’ll give an update to this post once I’ve given it a go.

When you push the brake fully is the extra safety cable tight or loose?

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I think it’s time for a video or some pics if you can post them.

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Loose, struggling to get it tight. Thought I’d just leave it as it is as it’ll still do the job if my brake rod fails.

I would’ve by now but I store my kart at the circuit which only opens on weekends :confused:

Ah, yeah that makes it harder. Well, contaminated pads and or disk should be recoverable .

Be careful that at full extension the cable doesn’t pull tight before the rod does. That’ll create a “stall” in the pull and result in not getting full brake pressure. Fits what you are describing.

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I will investigate this, thanks for the suggestion!

I think you have this backwards. If you move the attachment point up on the pedal, you will have less pedal travel and will not be able to apply as much pressure to the master cylinder. If you move the attachment point down, the pedal will travel slightly further and you get more mechanical leverage to apply greater pressure to the master cylinder push rod.