Worn out master cylinder?

[Mechanicaldan]

Hiya,

Just trying to find out if anyone has worn out a master cylinder before? My '98 ZX-9R is about to fail it's 3rd attempt at passing MOT because of front brake issues. First I replaced the brake lines because the front brakes were "mushy" and not considered safe. Did that, and then the brake caliper were sticking, so they got rebuilt. Brakes are still mushy. I just rebuilt the master cylinder with a new piston and seal kit, but it's still mushy. I've got a vaccuum bleeder, and the fluid comes out both calipers without air bubbles. I've bled almost a full container of brake fluid with no difference.

I can 'pump' the lever to built pressure, but it's still mushy. I'm just struggling to believe the master cylinder is wore out. There was some slight polishing on the master cylinder bore ID and the metal piston OD, but I though a new seal kit would do it.

So, anyone else wear out a master cylinder? Bike is 11 years old.

Thoughts or opinions are appreciated, even regarding a master cylinder upgrade.

Thanks,

Dan

[metallninja]

Sounds like the bleeding of the master cylinder could be a cause?

That is to say that it`s not enough just to bleed the brakes at the calipers, if the master cyl has a bleed nipple - start there.

Could be of some use or no use at all

[sneezey]

Had the same bother with my 02 ZX9, absolute nightmare to bleed. I ended up bleeding just one of the calipers at a time, that seemed to work but always had to top up the master cylinder when I was doing it. Used the jam jar method, too tight to buy a vacuum bleed kit.

And had a feckin nightmare with the caliper needing stripped and rebuilt every 6 months or so. Make sure that the calipers aren't binding, otherwise the front disks get heated up and distort and need replacing, not cheep!

[Backs 400]

Normally speaking, if the seal is not making good contact in the master cylinder you would get fluid leaking past the seal and dripping. This is more noticable if you "pressurise" the system and leave over night...to do this get a bungee cord, squeeze your brake lever as hard as you can and then bind tightly in the "on" position. Leave over night and check in the morning...oh..place rags under the cylinder and kitchen roll on top..if it drips you will clearly see the stain on the kitchen roll.

If there are no drips, have a good look around the master cylinder for weeping or moist "seams".

If nothing, then pretty safe to say the seal is doing its job and the issue lies elsewhere. Also, you may notice a distinct difference in your brake leaver after unbinding it. Should not feel so mushy for a while. I used to have to do this each night on my ZXR!

When you changed the hoses did you use new crush washers on EVERY join? Did you replace the hoses like for like or did you change the amount/arrangement of the hoses?

Have you tried removing the calipers and tilting them when bleeding? Sometimes air gets trapped in awkward places.

When the calipers where re-built are you sure all the seals where used and that they are seated correctly..again look for fluid seeping past onto the pistons.

[al]

Normally speaking, if the seal is not making good contact in the master cylinder you would get fluid leaking past the seal and dripping. This is more noticable if you "pressurise" the system and leave over night...to do this get a bungee cord, squeeze your brake lever as hard as you can and then bind tightly in the "on" position. Leave over night and check in the morning...oh..place rags under the cylinder and kitchen roll on top..if it drips you will clearly see the stain on the kitchen roll.

Sounds like you have prostate problems!