KDXRIDER.NET

Hi all
I'm doing a rebuild on my 90 kdx200sr calipers as they are sticking a bit. The pistons are the phenolic type, do these typically give problems as they get older? I'm just wondering if a seal kit would be adequate or if I should find some stainless steel pistons too.
Cheerd

Have you disassembled the calipers and looked at the bores? The pistons? If they look good they’re probably just fine but this is the first I’ve heard of a brake caliper having phenolic pistons.

The seals are 32 years old minimum. I suggest fully disassembling and inspecting.

I think the pistons are phenolic, though I am not too sure what that means.

KDXGarage wrote: 10:56 am Apr 28 2021 The seals are 32 years old minimum. I suggest fully disassembling and inspecting.

I think the pistons are phenolic, though I am not too sure what that means.

Phenolic is a composite type of material. I wouldn’t think it would be used for caliper pistons. It machines nicely though and is stylish way to make custom parts such as instrument panels.

https://www.jstor.org/stable/44467086?seq=1

The picture on this page shows almost exactly what a KDX rear caliper piston looks like. It's apparently phenolic, as I have had them out before, including a chipped one from some nimrod (previous owner)

http://www.safebraking.com/tech-why-are ... c-plastic/

Here's a pic, I assumed they were phenolic but I know they definitely are not metal.



The bores look good, nice and clean.
The front caliper pistons look good but the rear is showing some abrasion, it wasn't enough to cause a leak but I suspect it is the cause of the dragging pads. I have seen a few recommendations online so will try giving it a polish with some superfine paint polish I have and see how it turns out.

From my reading knowledge, the number one drag is worn brake pins. What is the condition of the pins on your calipers?

The pins have lost their zinc coating in parts, they are still smooth but will be getting replaced too.
When popping the rear piston out I had to turn up the psi on my air compressor as it wasn't coming out.

This is all new to me. I don’t think I have ever come across a caliper piston that wasn’t metal. Very interesting.
Like KDXGarage said, drag is primarily caused by the pins. If the caliper can’t float properly it will make them feel horrible. The caliper needs to be thoroughly and lubricated properly with brake grease to bring it back to it’s original glory.

Thanks for the info, I will make sure to grease the pins and fill the rubber boots too.

In my experience with a dragging rear brake, it was due to a gunk buildup in the caliper cylinder. Had a hell of a time getting the piston out.

Once all cleaned up and new seals and pins fitted, everything now works as it should.