Why does B-King's lever come back to the bar?

Q. I am having problems with the brake lever travel on my Suzuki B-King. When cold, with the lever set at full span it almost comes into the twist grip, with no resistance. But once I get a very little heat into the calipers I eventually get a good lever.

A new master cylinder kit has been fitted and I have tried all the bleeding tricks, using a pressure bleeder and moving thecalipers above the master cylinder, plus lever held in for 24 hours, but nothing changes.

When the brakes are not used for a short period, ie motorway or dual carriageway, the lever travel is excessive. This can be quite concerning!
David Morrice, Aberdeenshire
 
A. It sounds like you have some corrosion in the system, but it’s around the calipers and the brake discs, not the master cylinder. If the bike has been through a winter as harsh as the last one, it’s quite possible that corrosion has sneaked into the circumferential slots that the caliper piston seals sit in.

Those seals are angled back from the brake disc face so that they pull the brake pistons back a fraction when lever pressure is released.

That way thebrakes don’t bind, especially as they heat up. When there’s corrosion the seals swell in their slots and pull the pistons back too far, giving you that lever travel.

To double-check, clamp off both flexible hydraulic pipes (if they aren’t braided lines) using mole-grips down near the calipers. If the lever goes solid, you know there’s no air in the system.

While you are overhauling the calipers, clean out the disc bobbins that allow the discs to ‘float’ and self-centre when the pads start to grip them. If they have got crudded up, that can give you a soft lever.

Either remove them completely and give them a good hot water and washing-up liquid wash, or dothem in situ by threading an 8mm nut and bolt through each hole in turn, nipping them up, then use them to rotate the whole bobbin 20 times, spraying WD-40 around the place too, as the cack comes out.

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