Brake problem - GS500 sliding caliper issues, help please!!

[D O G]

Hello again.
Yet more AIDS with the front brake on the GS. It has developed a ‘dead’ zone for the first half of the lever travel, until it bites.

There is no spongyness when it bites, and I did a hopeful bleed the other day to try and cure the problem with no joy. There were no bubbles of air either.

I had a look at it yesterday, and it appears that the caliper is moving when the lever is pressed through this dead zone. The caliper is a sliding type, and the pins on which it slides locate into two different type of ‘holes’.
A diagram can be found here. You can see that the ‘top’ pin locates into a blind tube on the caliper body, and the ‘bottom’ pin into a rubber sleeve which passes through an eye on the caliper body.

The caliper moves only at the top slide – mounted into the blind tube, sliding out when the lever is depressed, and then back IN when the lever is released. It is this sliding back in bit which causes me the problem – as the pads move away from the disc.
My crackpot theory is that when I rebuilt the caliper recently, I stuffed the top locator full of copper grease, so when the slide has move away as the brake is used, a vacuum has been created in the tube, which causes the slide to return when not under pressure. Is this a stupid idea?

What else could it be? There are no leaks from the caliper or seals, or banjos or whatever. Could it be the master cylinder, which is the only bit which has not been replaced pretty much!

Any help would be appreciated.

Thanks!

[geordiecbrblo...]

I'd be tempted to remove the caliper off the fork and make sure that the sliders are free - my CBR600 has the same front calipers and the sliders to seize up regularily.

I'd say it is possible that the excess copper grease is causing the sliding part to pull back, so take it off and make sure it is free

[Kickstart]

Hi

It should slide on both pins pretty smoothly. Often not THAT easily though (ie, don't expect the caliper to just fall off the carrier).

All the best

Keith
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[D O G]

Well I've taken it off, cleaned it up, spot of Copper Grease later and still no change.

I reckon the big piston is sticking.

Not amused one bit.