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Offline DesertPilot

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Rear Brake cleverly stopped working
« on: November 20, 2019, 10:07:57 PM »
Plenty of wear left on the pads, plenty of fluid in the reservoir, but the pedal goes all the way to the... well, I guess you can't call it a 'floor' but you know what i mean... without accomplishing anything more than giving my right foot a workout.

I'm guessing -- increasing of of 'loathsome-to-repair-ness' -- that either there's air in the system, the seals on the master cylinder have given out, or the slave cylinder has frozen.  Frozen slaves: a evil warlord's nightmare.  But I won't be able to get around to pulling things apart to have look for two weeks, so we'll see what I find then...
Still trying to master the use of fire and learn how to fashion primitive stone tools

Offline Grumps

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Re: Rear Brake cleverly stopped working
« Reply #1 on: November 20, 2019, 10:38:21 PM »
I think a frozen slave would result in a rock-hard pedal. Sounds more like a knackered master cylinder if no fluid leaks in the system
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Offline DesertPilot

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Re: Rear Brake cleverly stopped working
« Reply #2 on: November 21, 2019, 12:16:24 AM »
That was my thought as well.  When I bleed the system, I'll thaw out one of the slaves and have them... oh dear... pull off the gaiters to check the master for leaks  :002:
Still trying to master the use of fire and learn how to fashion primitive stone tools

Offline DesertPilot

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Re: Rear Brake cleverly stopped working
« Reply #3 on: November 21, 2019, 12:30:16 AM »
No sign of any leaks, the pedal doesn't feel spongy, and the brake does stop the wheel from spinning when the bike's up on the center stand.  That suggests the system's basically healthy.  Maybe the pedal's badly out of adjustment.  This may take some fiddling, but I doubt it will be a big problem.  I'll fiddle with it after my wife and I get back from Australia.  Where I imagine they ride Regits, because that's in the Southern Hemisphere...
Still trying to master the use of fire and learn how to fashion primitive stone tools

Offline DesertPilot

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Re: Rear Brake cleverly stopped working
« Reply #4 on: Yesterday at 11:44:24 PM »
I've sorted the problem, and I'm posting the solutuon so others can benefit from my iexperience and/or oblivion to the obvious.

The Symptoms
1) The rear brake worked, but was weak
2) Length of the master cylinder pushrod at full extension had increased from the specified 75.5 mm tp 80 mm or so
3) The pedal felt firm, which suggested the problem was not air in the system
4) When I inspected the system, there was no sign of leakage
5) Everything else seemed fine.  The pads were good, brake fluid level was normal, sun rose in the morning, there was beer in the fridge, etc..

Repair attempt #1
1) Assuming that the rear caliper had gotten gummed up and begin to freeze, I cleaned it very carefully with brake system cleaner with the caliper still on the bike, pulled it off, taking care not to compress the piston and risk pushing dirt past the seals, continued cleaning it until it was spotless, then compressed the piston to make certain it wasn't sticking.  No problems there.
3) I scored the pads and rotor very lightly with 400 grit wet/dry sandpaper to remove any glaze, then cleaned the heck out of them to remove any dust.
3) I put the everything back together, being sure to grease each and every one of the grease points with brake system grease
4) Since the brake pedal travel still seemed excessive, I adjusted the nuts on the fork at the bottom of the pushrod to raise it.  NOTE: It's easy to bend that fork when tightening and/or loosening those nuts.  The best way to avoid this was to hold the fork in place with an adjustable spanner.

Result of Repair Atempt #1
The brake was better, but still weak

Repair attempt #2
1) I bled a very slight amount of fluid out of the slave cylinder -- just a few cc's, not a complete flush.  I found it advisable to cram newspaper around the fluid reservoir to intercept the inevitable spills, and remove the seat to make it easier to fit a (freshly cleaned) funnel to add brake fluid.
2) After I'd pumped in a few cc's of brake fluid, the pedal went from 'firm' to 'rock-hard firm'.  I closed everything up, then adjusted the pushrod length back to 75.5 mm.

Result of Repair #2
Is fixed

Conclusions
1) "Trust your feelings, Luke.  If you feel a disturbance in the Force, as if the brakes need bleeding, they probably do."
2) The rear brake pedal shouldn't merely feel 'firm', it should  feel 'rock-hard firm' -- significantly firmer than the pedal on our Ducati.  This may reflect some difference between the English and Italian character.
3) If if doesn't, flushing a small amount of brake fluid out of the slave cylinder could do the trick.
4) The 75.5mm specification for master cylinder pushrod length at full extension seems correct.

Remaining mysteries
1) What was wrong?  I didn't spot any bubbles or other sign or air in the system.  I also didn't spot any sign of leakage.  Was there dirt in the system?  Moisture?  Did some valve in the ABS or master cylinder need fluid forced through it to get it unstuck?  We shall see.
2) Did pumping a few cc's of brake fluid through the system confuse the ABS?  See note above about 'we shall see'.
Still trying to master the use of fire and learn how to fashion primitive stone tools

 


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