Caliper / Master Cylinder Ratio's

[spannerman]

i have a small problem which i believe must be a master cylinder ratio issue, basically i have a zx6r g1 the six pots have been replaced by nissins from a bandit 1200 i believe. i have been bleeding it for a couple of hours and next to nothing the master cylinder is the standard one from the zx6 5/8 is what is cast into it.

is it not pushing enough fluid to work the pistons? the discs and pads are not in great condition but there is no lever and it only just locks the free wheel, you can't even pump a lever. and the fluid appears to have no air in.

[Mark/Foggy]

Oh God here we go again.

It's line locked somewhere, or the mastercylinder has decided to spit the dummy now, while you've been messin with it. Don't ask me why, but they can choose to do this. Had a brand-new Beringer do this to me in the last month.

If you've got pistons out a bit, get them off the bike and hang them up, above the master cylinder. Try bleeding again with the nipples as the highest point. Knock the calipers from time to time and watch the pistons. If you're air locking in the caliper, a slow piston could point to where the air is. turn the calipers over allowing any air pockets to move under the nipple and bleed again.

When you've got all the pistons moving the same, crack the banjo at the master cylinder , wrap with rag and push the pistons back in the calipers, This will run any airlock at the master out of the system. Relock the top banjo and push the pistons back until they're fully home, you should see the fluid rise in the open topped resevoir. If you start seeing a thin trace of bubbles at this point, in the resevoir, could be that you've got a leaking seal in the master.

[spannerman]

Cheers I'll try that see how I get on

If I can't get it I'll chuck it in the Tyne and report it stolen

[Wildbill]

Do not throw it in the Tyne; there is enough shite in there to start with.

Throw it in the Wear near Sunderland; they appreciate such things !

The reason master cylinders seals often fail when bleeding is; the piston has previously only travelled a short distance

inside the cylinder and pressurised fluid helps to retain the shape of the seal.

When bleeding, the full stroke of the piston is utilised and the seal fails or distorts.

Leaving a spare cylinder to drain out in the stores accentuates this.

[lorenzo]

I've found that if you start by bleeding the master cylinder first (use a rag and crack the banjo open) you tend to do away with the toughest bit of bleeding the system, which is getting the fluid through the whole system in the first place.

Of course, if you know anybody with a mityvac bleeder, then that bit would be done in about 45 seconds, they're awesome.

[Mark/Foggy]

Do not throw it in the Tyne; there is enough shite in there to start with.

Throw it in the Wear near Sunderland; they appreciate such things !

The reason master cylinders seals often fail when bleeding is; the piston has previously only travelled a short distance

inside the cylinder and pressurised fluid helps to retain the shape of the seal.

When bleeding, the full stroke of the piston is utilised and the seal fails or distorts.

Leaving a spare cylinder to drain out in the stores accentuates this.

That my friend is a good answer. Never got around to thinking it through.

[TLRS]

Once you have some pressure in the system, try using (clean!) pistons to pump. You have to make sure they go in straight. You also have to make sure you have enough fluid to push some pistons a good way out first. Then keep an close eye on the fluid level once you start using pistons to pump fluid back up.

This will flow a lot of fluid. This should help out any air trapped in bores that are joining both halves of the caliper. Or other bits that a place that revents the air from moving up, like a hose running over the muddy joining the calipers. Another way is to unbolt the calipers and put them on their side. Again to enable air to move up.

Sometimes the shortest route from the m/c to the bleed nipple isn't through the entire caliper. So using aforementioned method might help remove more dirty fluid. And once it is clean, you can pump it up and down the system with no need to bleed. Keeping wasted fluid to a minimum.

If you've got pistons out a bit, get them off the bike and hang them up, above the master cylinder. If you're air locking in the caliper, a slow piston could point to where the air is.

The first can be awkward on anything else then naked bikes. The second thing is not true as far as I know. Pressure is equal in the whole system. I think a 'slow' caliper has more friction, requiring more pressure to move. That pressure is not enough to move that one piston, but plenty to move others.

Edit: a picture to show how some calipers sides are joined and air might get trapped.