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Rear master cylinder swap - what am I doing wrong?

2K views 4 replies 3 participants last post by  RampantParanoia 
#1 ·
I got sick of the terrible rear brake on my bike and decided to swap the master cylinder out to a 1/2" bore one as D'Ecosse has recommended to various people. I got a cylinder second hand from a 1991 ZX750 off eBay and put it on the bike, then refilled through the caliper bleed nipple and bled the system.

Despite bleeding, leaving overnight, etc, I couldn't get it to firm up at the pedal, it literally just went straight to the end of travel with no resistance at all, and didn't do anything the rear wheel to stop it moving.

I figured the seals were shot so I replaced all the internals with brand new genuine Nissin replacements and checked the bore for scoring/corrosion etc but it looked fine. I filled it up again through the bleed nipple but the pedal action is still no better and no matter how much I pump (counted at least 30 in a row) it I can't get it to build any pressure.

Is there something I've missed or done wrong? I thought this should've been easy!
 
#2 ·
I was in a similar position after stripping and repairing my rear caliper. Kept bleeding it and bleeding it but could not get a pedal. I then bought a Vac Bleeder based on recommendation but still couldn't get it to bleed properly.

I removed the caliper and held it with the nipple facing up, pumped up the Vac Bleeder, cracked the nipple and a load of air came out. Got a firm pedal straight away. It seems the internal design and the way the caliper is mounted means air must get trapped in there somewhere.

Give it a go and see how you get on.
 
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