When was the last time you cleaned out your Master Cylinder?

Joined
Aug 6, 2015
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328
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Peoria, Illinois
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1995 ST1100
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8866
I'm really curious to do both the brake and clutch master on mine after working on my brother-in-law's CB750 Nighthawk. I was shocked at how much gunk was in his bore, even though we had flushed the master and the caliper thoroughly during the refurb. When I removed the master cylinder piston rod, there was a lot of black gunk even though the reservoir and lines were clean. Also, there's a TINY relief hole in there that that you need to be certain is clear.

I'm looking into master kits and will probably tackle it on my ST this fall. This is just a potential goo-alert in case you've never opened yours up.
 
Joined
Aug 10, 2015
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Fort Worth, Texas
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91 ST1100/06 ST1300
Even some 'rebuilders' ignore this fact. I suspect it's because they don't know. There are static regions in most master cylinders where fluid will not flow. Not really trapped, it just isn't moved by the movements within the system. When it starts to gunk up, the assembly will probably work fine...until little bits of the growing goo deposit start to dislodge and finds a home where it can inhibit fluid flow.
Always take it off the handlebars and clean the whole assembly during a rebuild. No real clear guidelines between rebuilds that I've ever seen.
 
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Feb 2, 2009
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Dahlonega, GA
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2018 NC750X
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7666
I've helped some folks clean and flush the clutch and brakes on their 1100's and 1300's. The worst case was an 03 1100. I literally removed the main portion of gunk from the reservoir with a screw driver! Yes, it was that bad!
 

ST1100Y

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I always push the calliper-pistons fully in during brake fluid replacement, suck the reservoir fully dry (some debris on the bottom, don't forget to lift the little SS-plate), then I flush with lots of new brake fluid (having one of those automatic, air-powered vacuum-kits does help though, got my hands free to keep the reservoir filled while the device draws the liquid out the bleeder nipple)
This stretches my overhaul interval to like 5 years...
Whenever I sense significant increased resistance while pushing those pistons in (like when replacing the pads), the callipers get taken apart and cleaned/rebuild ASAP, cause neglecting that symptom can lead to dragging brakes, yet seized pistons real quick.
 
Joined
Sep 10, 2011
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West Michigan
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'98 ST1100
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8470
I bought a spare set of calipers ( front & rear ) off Ebay , disassembled and cleaned them including cleaning the "build-up" in the piston and dust seal bores using toothpicks.

Every time I install a new tire, I swap calipers and then do the same to the calipers I pull off the bike. So I never have to worry about sticky caliper pistons and the brake fluid is regularly flushed and replaced.
 

ST1100Y

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Every time I install a new tire, I swap calipers...
Now that's slightly extreme and extravagant... :lol:

But we've different scales there, since my tires will give a max of ~12.000km/7.500miles, whilst you guys wear them down to the steel belt... ;-)
 
OP
OP
dkruitz
Joined
Aug 6, 2015
Messages
328
Location
Peoria, Illinois
Bike
1995 ST1100
STOC #
8866
Even some 'rebuilders' ignore this fact. I suspect it's because they don't know. There are static regions in most master cylinders where fluid will not flow. Not really trapped, it just isn't moved by the movements within the system. When it starts to gunk up, the assembly will probably work fine...until little bits of the growing goo deposit start to dislodge and finds a home where it can inhibit fluid flow.
Always take it off the handlebars and clean the whole assembly during a rebuild. No real clear guidelines between rebuilds that I've ever seen.

Absolutely - the piston and bore of the master cylinder don't get much "flow" once the brakes are bled. We did his thoroughly (twice) and thought everything was good until his front caliper started to bind. Turns out that miniature relief hole plugged and wouldn't let pressure off. When the caliper warmed up, the brake fluid expanded and with no-where to go it caused the front to bind. Simply poking the hole in the MC clear *could* fix it temporarily, but opening it up was the way to go. We didn't need to replace anything to get it working, just clean and reassemble.

Simply seeing what was out of sight and trapped inside the master cylinder makes me want to open mine and clear them both out.
 
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