Rear brake issue (cont'd) - please help me troubleshoot this - Kawasaki Forum ::
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post #1 of 4 Old 04-14-2019, 04:40 AM Thread Starter
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Rear brake issue (cont'd) - please help me troubleshoot this

Thanks in advance for your time and advice. After thoroughly cleaning and then rebuilding the rear caliper (polished the original pistons, installed new o-rings in cleaned/clear o-ring grooves) and the master cylinder (new piston, rubber seals and spring), the rear brake pedal and brake operation felt great. Applying immediate pressure to the rear pedal would slow and stop the bike.

However, after a short ride (2m = 50mph), the bike again slowed to a stop, with the rear caliper clamped down on the disk and the brake pedal nearly "locked" in an applied fashion. I had to crack the rear brake line at the caliper to relieve the pressure to get the bike home. Before the brake locked up, I used it 7-8 times testing the brake operation (solid). I ride with my right boot definitely off the rear brake pedal when not using it.

A few things:
  • Same end result 3x in a row (1) On first ride after picking up the bike from the transport company, which destroyed the rear caliper. (2) on second ride a week ago after disassembling and clearing a nice used rear caliper and brake disk (+ original master cylinder). (3) on third ride after rebuilding the rear caliper and master cylinder.
  • The brake pads and disk were dragging a slight bit (before my my second and third ride). Is it possible the caliper is heating up causing the fluid to boil = pushing the caliper pistons out of their bores?
  • Looking back on my caliper/master cylinder rebuild yesterday, I didn't ream out (with a steel guitar string) any of the tiny passages in the master cylinder. After cleaning, blew compressed air through all ports as much as possible. Also, I didn't ream/clean out any narrow fluid passages in the rear caliper other than the bleeders. Wondering if I missed something with both components?
  • The only other thing I can think of is a weak rear brake hose that may be collapsing internally.

What do you guys recommend? I'll try anything different at this point. Thank you.

2000 ZX12R (A1) with 14k (Candy Persimmon Red)
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post #2 of 4 Old 04-14-2019, 11:27 AM Thread Starter
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Dang... think I got it fixed. I spent this (Sunday) thinking through what keeps happening and what I've not yet done. Then it hit me -- I didn't clean the return on the rear master cylinder! So off she came again (i'm getting really good at disassembly, btw) and drained the reservoir, removing the circlip from the plastic hose inlet-to-master cylinder opening. Man was that dirty. Ran an .010 high E guitar string through there and cleaned with a toothbrush and brake fluid. Reassembled the bike after a bleed and rode 5 miles without ever touching the back brake pedal. No issues/no dragging. That eliminated any light dragging concerns. About 1/2 mile from home, I used the brake pedal about 25x. No sticking or slowing. Think that's it! going to take another quick ride after I get the garage cleaned up. I wasted 4-5 hours (probably more with research) jacking with this because I didn't take an extra 20 minutes to fully address every avenue.

Now I need to take her on a spirited run. But MAN is that Muzzy titanium exhaust loud. Not sure I want to annoy half the town alert every policeman every time I hit 5k rpm. The seller gave me the original exhaust with the bike...we'll see.

2000 ZX12R (A1) with 14k (Candy Persimmon Red)
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post #3 of 4 Old 04-14-2019, 01:25 PM
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glad you got it sorted out. i was going to suggest checking the rubber lines as ive seen them come apart internally (on cars only so far) and act as a check valve trapping pressure in the caliper, but it sounds like it was just a bit of gunk doing the same thing in a small passage.

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post #4 of 4 Old 04-18-2019, 05:43 PM Thread Starter
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Thanks, KZScott. You are exactly right -- gunked up. The original owner told me he NEVER changed the brake fluid. My initial bleed obviously didn't do the trick.

I was lazy and paid the price of time lost. One would think with as many years as I have turning wrenches on bikes and cars (as hobby) and fixing all kinds of stuff (as a hobby and to save money) I'd learn it pays great dividends if you take your time and cover all the bases. Doh!

I'm just thrilled to have the bike starting, running and stopping like she should.

2000 ZX12R (A1) with 14k (Candy Persimmon Red)
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