Those Tokico Six Pots, and Other Brake Ramblings
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Thread: Those Tokico Six Pots, and Other Brake Ramblings

  1. #1
    Baby Twin
    Join Date
    May 2018
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    Hayward, California, USA
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    Those Tokico Six Pots, and Other Brake Ramblings

    I had a pretty successful weekend, all things considered. But it's given me pause for thought about the brakes. I'm going to write what I think; if you disagree, that's great but I reserve the right to agree to disagree. My thoughts are without warranty express or implied.

    I rebuilt those dreaded six-pot Tokicos without any real difficulty. The pistons were pretty gummed in and the dust seals were pretty wedged up on more than one of the pistons. Some judicious squirting of air down ports got them out anyway with hardly any injuries sustained from flying pistons.

    The pistons weren't immaculate but they seemed to clean up ok with a wire brush in a drill. The finish on them is obviously pretty hard because the steel brush didn't touch them except to get the congealed seal crap off them. The dust seal grooves were in a shocking state with a crusty white deposit that was quite recalcitrant, but since I wasn't fitting the dust seals I wasn't that bothered. I gave the main seal grooves a clean - they weren't too bad - with something less aggressive in a dremel. I note the main seal grooves are a more elaborate design than I have ever seen in any other caliper. My day job means I look at a few.

    I put them back together with only the pressure seals, lubricated with brake fluid, then bolted them back onto the bike. I did the "finger over master cylinder banjo port" trick to be sure the master cylinder was properly flooded, then attempted to bleed them using what I will call a "traditional" technique. For the left caliper this is pretty awkward on the TL, requiring a slightly obscene stance over the front wheel to reach the bleed screw and brake lever simultaneously. I noticed, looking in from the front of the bike, that the outboard pistons on the left hand caliper (right as you look from the front) didn't seem to want to energize. I dropped the caliper off the fork leg and with a Heath Robinson combination of brake pad and two zip ties, held in the inboard pistons while pumping up the outboard ones. The middle one seemed determinedly reluctant, but in the end woke up. back on the bike, all four sets of pistons seemed to energize in a broadly equivalent way after that.

    The traditional bleed didn't seem to be working that well, and was where I put things down at the end of Sunday 29th (?) to order a MityVac. I got the ~$70 metal one as opposed to the $20 Harbor Freight competitor. Last Saturday I plugged in the MityVac but didn't notice a huge change in the quality of the bleed in terms of feel at the lever at a standstill. Hmmm. I did that thing you should never do, and zip-tied the lever compressed. On Sunday I put the pipe on the bleed screw and with the zip tie still in place, cracked the bleed port on the left caliper (while squeezing the lever to make it go all the way to the bars) then closed it down again without releasing the lever. This made a pretty satisfying noise but I wasn't seeing any evidence of air in the bleed tube.

    The reason I mention the MityVac is that I also rebuilt the rear caliper. It's a long story but due to basic ineptitude I ran the piston (yep, the piston) against the brake disc for two stop signs. The other side of the disc was treated to the backing plate of the pad that should have been between piston and disc. So, a new disc and new pistons were required, which is basically another full rebuild on the rear caliper - caliper off, split the halves, etc. Basically a totally dry build before reattaching to the bike. Cue MityVac. A single cycle on each bleed port and the rear brake came up pretty much perfect. No, really - once everything bedded in after a few miles, the rear brake feels like a honey.

    So this got me thinking. What's happening on the front?

    I have a theory.

    These 6-pots are well known for not being super robust. I think that over time, one or more pistons gets jammed up - probably as much on those cursed dust seals as anything else. This has the effect of shortening the lever, because there is less fluid to displace. After a rebuild (and some odd coaxing on my part) all the pistons are free again.

    I think this is why it feels like the 6 pots don't bleed up properly.

    Riding to and from work today on all three rebuild & bled calipers, the brakes felt better than they have done so far. In fact, they feel better than the brakes on my '88 VFR (as they should) and really very credible compared to the super new, electrickery brakes on my BMW K1600. They have progression, bite and an ability to modulate them - almost like a real sportsbike. This came on over a period and wasn't immediate - I think there is something going on with the pistons settling in the seals, too, after some miles of running.

    So, that's my theory. I think the 6 pots feel as if they don't bleed up properly after a rebuild because they were seized before the rebuild. Flame on.

  2. #2
    Baby Twin
    Join Date
    May 2018
    Location
    Hayward, California, USA
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    42
    Oh, and the fuel economy has jumped up from about 33mpg US to about 40 mpg US. The bike is noticeably easier to push around. I think the brakes were in a pretty sorry state before. I hang my head in shame that it took me so long to get round to rebuilding them.

  3. #3
    WSB Pit boss tlsgazza's Avatar
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    May 2004
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    wellingborough, northants, UK
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    5,636
    I've just replaced another set of badly warped front discs. My 6 pots weren't too bad I thought so I pushed the pistons back in to put new pads in too after a scrub with brake cleaner.
    First ride out and they were obviously dragging just a little as the new discs were a little bit hot. The brake pads were dragging a little on the paddock stand when I got home.
    Yesterday I dropped the calipers off and gave all the pistons a really good clean up with them pumped out as far as I dared. Pushed them in and pumped them back out a few times and they do seem to be a lot better. The wheel spins much better on the stand than before and I can feel a big difference just pushing the bike in the garage.
    I will give it a run out in a couple of days when I get a new flasher unit fitted so the indicators work again. The last one dropped down and melted itself to the rear header pipe just below it .
    Anyway it was nice to ride again without the warped discs so I felt I could actually use the brak s again.
    black TL1000S rider,owned from new since august 98,hinkle windowed clutch cover,braided lines,5/8 front brake M/cylinder.1/5 throttle,Helibars,double bubble,stainless renegade cans, ohlins damper,stainless rad and oil cooler covers,undertray,carbon hugger,silver powdercoated wheels,k+n filter and TRE,R+G bungs,LED indicators,gold X ring chain with 40T rear sprocket,mini indicators, HID H4 headlights with led side lights.now done 85,000 miles.
    gold busa forks,gold six pots and gold rear caliper,pazzo levers all thanks to speedking
    van driver clutch pressure plate and bolt retainers,
    also done 306 laps of the nurburgring,
    now have my r1200gs adventure with 95.000 miles covered to date.

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  5. #4
    Baby Twin
    Join Date
    Aug 2013
    Location
    Brisbane
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    Freshly rebuilt with decent pads and a radial master cylinder (3/4 Nissin) they will do 1 finger endo's cold but thats where it ends, they terrible at track temps with inconsistent lever travel, plenty of theories as to why but basically the only solution is replacement. The other downside is corrosion, the bores aren't anodized and corrosion builds up behind the seals, Nissin 4 pot calipers have anodized bores and dont suffer this issue or fade.

  6. #5
    Baby Twin
    Join Date
    May 2018
    Location
    Hayward, California, USA
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    42
    The corrosion was fairly obviously the problem with my calipers around my dust seal bores jamming up one or more of the pistons. I may yet regret the decision to leave the dust seals out (though I may not).

    I can't comment on the track performance, but it does occur to me that another solution is not going on a track. That's the one I'm experimenting with at the moment. ;-)

    In my job as a brake engineer I've never really known calipers themselves to be the cause of fade. It's more like temperature at the pad/rotor interface - the friction coefficient drops and drops and you end needing to clamp really hard to generate brake torque, until in the end you either set fire to the resin in the pads or structurally compromise them. If I were experiencing fade the first thing I would go with would be thicker rotors. Another 1mm will bring temperatures down by about 20%. I have no idea if anyone sells them but that would be where I would start.

    Excess heat can and does work its way into the fluid and can boil it if it's not fresh, but that causes the brake hydraulics to effectively collapse - it's not "inconsistent travel", it's just no brakes. I recently ran a brake test that got the rotors to 640deg C but there was no hydraulic collapse - caliper fluid temperatures were well below the atmospheric pressure boiling point on fresh fluid. The pads are pretty good insulators and only about 10-15% of the total thermal energy goes into them, the rest is in the rotor. Both inconsistent travel and fade can come from contamination around the recuperation port in the master cylinder by effectively trapping a precharge in the brake system.

    Personally I think there is also something a bit weird with the brake seal groove machining detail, as I commented on in the original post. I think it causes some strange pressure distribution when you energize the seals. I bet that the Nissins have a regular rectangular seal groove - the last ones I rebuilt did, and the AP Racing calipers before that. And the corrosion as you mention. I'm kinda surprised to hear they're not anodized.

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