OTK BSM rebuild help / piston not moving

I am in the process of refreshing my daughter’s new (to her) 2014 FA Rookie. I need assistance with a few things. Thank you in advance.

The caliper is an OTK BSM model.

The engine-side piston is not moving. Given the design, it looks like it should be a two-piston caliper, so I figure something is wrong. The below picture is the brake at rest. When the pedal is pushed, only the non-engine side pad moves. There is a pretty substantial gap between the pads and the rotor on that side.

The pedal throw is almost all the way to the bumper. I’m assuming a bleed would be a starting point for that. I don’t however want to get ahead of myself if I should be pulling the brake apart anyways to deal with the stuck piston.

I don’t know what the master cylinder compression rods (right description?) should look like - neither when compressed or not. I’ve added pictures below to show both statuses. They are not compressing equal, one side does more than the other (it pivots). Is this normal?

  1. Am I correct that it is a 2-piston system and one is clearly stuck?

  2. If it is clearly stuck, is this something where a bleed will likely sort it out? Or should I be getting a rebuild kit?

  3. If it’s a rebuild, does anyone know where to find good instructions? I’ve been struggling to find good resources for thing like this, as the kart is a bit older.

  4. On the topic of a bleed, is there an OTK specific kit for the BSM brake system? If not, any recommendations?

  5. Should bleeding correct the throw issues with the pedal?

  6. How much gap between the rotor and pads is normal for a properly bled brake?

Thank you in advance. I can wrench on bikes and cars, but karts are new to me. Thanks!

1 Like


  1. Yes it is a two piston caliper

  2. I would bleed the system before doing any type of rebuild as they can be tricky. Power Republic has videos on both brake bleeding and brake rebuilds on their YouTube channel.

  3. May have preemptively answered this, but YouTube has good videos that will apply to older karts too.

  4. Tony kart makes a brake bleeder to aid in bleeding, the newest power Republic brake bleed video also shows how to bleed them with a siracha bottle but it’s not as thorough and can be tedious as Tony Karts brakes require more steps to properly bleed them.

  5. Bleeding the brakes will make the pedal feel a little stiffer than it probably is now, it’s completely normal and a solid feel when your press on the brakes is what it should normally feel like.

  6. I have not ever heard of a gap between the pad and rotor, however what I do is put the new pads in and loosen the bolts to the brake disc hub and then press the pedal a few times. This should center the hub and disc and ensure an even gap between the rotor and pad on both sides.

Here is the brake bleeder tool, they have been in and out of stock for a while due to supply issues but I bet you could find someone at your local track that has one and could help you bleed them. A telltale sign that you need to bleed the brakes is when both brake rods don’t compress evenly, as shown in your attached picture.

Here is the bleed video

And the rebuild video (if needed) it should be the same Or very similar to the BSM


The BSM system uses a spring return caliper as distinct from the almost universal automatic adjusting calipers these days. Pad to rotor clearance is adjusted by shims fitted between the pad and the caliper piston, about 1mm clearance per side.
The rotor on your kart is out of centre in the caliper. Usual reason is that the axle has moved sideways under cornering load. Check the grub screws in the axle bearings for movement and centralise the rotor.Shim or replace the pads to give 1mm clearance each side.
At the front, disconnect the master cylinder operating rod from the pedal. Both m/cyl push rods should return fully and equally and the connecting bar should be at right angles. Adjust the length of the operating rod to reconnect to the pedal without operating the m/cyls.
Now try the brakes. Hopefully you will have a firm pedal with reasonable travel and both pistons moving.
If not now is the time for bleeding or remedying.
An OTK kart of that age may well have silicone DOT 5 fluid in it, rather than glycol DOT4. DOT 4 is soluble in water. DOT5 is not .Take a sample and use the same fluid!!


I find this amusing. Ideal for bleeding the brakes on a Cayenne. Oh the puns.

1 Like

Is the best way to just drain some and pour water into it?

Just put some water in a clear glass and add your fluid sample.Only needs a drop of fluid.
Most karts these days have reservoir master cylinders which won’t miss a drop of fluid. I use a bit of small bore nylon pipe to get a sample.

1 Like

Sorry to bring this back up again. Maybe I’m just an idiot, but I can’t find a bleed nipple for the life of me.

Is there just not a traditional bleed nipple setup on this thing?

When I look at the blow-up below, it looks like M is the bleed screw. Should I be using a syringe or is there a threaded syringe out there that I need to acquire too?


Edited to add blow-up

1 Like

Bleed screw is the allen cap head above the feed hose connection.

1 Like

So it just actively drains fluid (vs draining into a bottle)? Weird process.

1 Like

Yes. Weird like a lot more things on karts!

1 Like

Messy, but it gets the job done.

Secondary note, because I was being an idiot - make sure that you follow the brake hose back to the caliper and are trying to bleed the right side.

Thanks everyone!

1 Like