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Bleeding new clutch master cylinder

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Replacing the clutch master cylinder on a 04 ST4s ABS. Specifically replacing the stock OE master with a Brembo Radial master cylinder from a '14 Multi Pikes Peak. (see photo)
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I thought I knew how to bleed a clutch but maybe I'm missing something... Using a MittyVac pulling fluid through the line I'm getting a steady flow of fluid without bubbles. However, when the system is closed I'm not getting any pressure at the control lever.

(The bleed screw on top of the master cylinder in the photo is where the OE reservoir went)

What am I missing? Did the exact same process with previous (axial OE) master cylinders with success. Can't figure out what I'm missing.

Suggestions?
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Discussion Starter · #5 ·
You can also try pulling the lever half-dozen times, then tie it back and crack the bleed nipple at the master quickly. Just be prepared for potential dribble or spray, depending.
I've been doing that also. Actually, my technique has been to alternate between using the MittyVac and then pumping the lever. I have a catch bottle on the bleeder nipple so the mess is kept to a minimum.

A challenge to the MittyVac is, as the tube to nipple is not a complete seal, there are always some air bubbles making it rather challenging to determine if the air is coming from the clutch line or the leaking seal at the nipple.

I'm still not getting much pressure at the lever, however. In fact, I don't really get much back pressure on the lever until it's pulled almost all the way in. And that with the lever adjusted for maximum master cylinder travel. On the center stand, engine running, it goes in and out of gear easily but it's clear that with the bike in gear and the lever pulled in (i.e. clutch should be completely disengaged) there is still significant clutch engagement.
 

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Discussion Starter · #10 ·
Hopefully someone more knowledgeable chimes in here, but my guess would be down to the difference in bore size on the master. If I recall, stock MC's on the ST's were either 12mm or 13mm, where the MTS is 15mm. Is your slave stock?
The stock ST MC is 12mm. I'm uncertain of the MTS size. However, I have an Oberon slave on it which is a 29mm i.e. larger than the ST OE which I don't recall the size (there's a chart of MC vs Slave sizing here somewhere)

The MTS MC should be a better fit for the Oberon slave than the stock OE slave.
 

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Discussion Starter · #11 ·
I Must admit, not for a moment did I think the bore sizes might be different.
You never mentioned it and I never asked. It’s just something I felt was a given.. same size equal cylinders.
See my previous comment. I went into this knowing of the difference in MC/SC sizing. That said, the sizing does not affect the bleeding of the system. Ya either get the air out or you don't. Apparently, I'm not getting all the air out.
 

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Discussion Starter · #12 ·
to stop air been sucked in around bleed nipples , first clamp small supply tube between clutch resivoir and clutch master cylinder. then remove bleed nipple on slave cylinder. dry nipple threads and apply Teflon plumbers tape . avoid tape on first two threads so no tape goes into system .Do same on master cylinder. Then use miry vac to bleed system .
You misunderstood me. I didn't mean air was leaking via the threads on the bleed nipples, I mean there is air leakage at the MittyVac tube which you put over the nipple (see photo). During bleeding, I would have an 8mm box end wrench on the bleeder screw below the plastic tube so the tube can't go down any lower. I have tried using a zip tie on the tube but that doesn't seem to make the tube seal to the bleed screw any better.
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Discussion Starter · #16 ·
do you have an original lever to try in it? or put as much freeplay into the one you have as you can.
If by original lever you mean the stock MC, then yes, I have it, but it was being problematic. Which is why I changing to the MTS MC. I'm not following what you mean by "...put as much freeplay..."

Currently, the MTS MC has CFG adjustable lever on it. I have it set for the longest travel. If I adjust it in for less travel, there isn't enough travel of the plunger to feel any back pressure.
 

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Discussion Starter · #17 ·
UPDATE:

This afternoon I checked it and there was back pressure when pulling the clutch lever. I found this a bit surprising. So I decided to attempt to bleed it ONLY with the pump-the-lever method. I put the catch bottle tube on the nipple, pumped the lever several times (4-7), held the lever closed on the last pump and cracked open the bleed screw. I got a nice flow of fluid sans any air bubbles. I figure this is a good sign. I tighten up the bleed screw, and then release the lever. However I immediately check the lever and there is no back pressure. I pump it several times and the back pressure comes back. I go through this cycle (pump up the back pressure, open the bleed screw, get a nice flow w/o bubbles, tighten the bleed screw, check the lever and there is no back pressure until I have pumped it again several times)

I take the bike for a ride, all seems to be operating correctly but I do gradually lose the back pressure on the clutch lever and I can't get the transmission into neutral. I pump the lever several times and the back pressure comes back and clutch/tranny work as their supposed to.

So WTF is going on here? Bleed the system and get all fluid with no bubbles should indicate the system has been bled completely. But by virtue of gradually losing the clutch should mean there's still air someplace in the system.

WTF am I doing wrong?
 

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Discussion Starter · #25 ·
9 times from 10 if it pumps up to work then goes away until pumped up again.. it is adjustment.. think brake drums on your car.

so your master might be smaller giving better pressure but not enough volume for your slave.

those slaves generally take more volume
Thx for the continued input.

When I first got the bike it had the stock MC and the Oberon SC and while it required more lever travel everything worked more or less fine. And worked fine for a couple of months until I changed the Oberon SC to the Stock SC. This combination (stock/stock) worked fine for a few months until one day, having just left the house and gone maybe .5 mile, I grabbed the clutch and there was nothing there. No warning, no going soft, nothing. Had just gone through maybe a half dozen shifts, went 1/8 mile, grabbed the clutch to downshift and there was nothing. I got it working again by bleeding the system w/MittyVac although other than lack of lever back pressure it didn't seem like anything was wrong.

At that point I changed back to the Oberon SC and again, all worked fine for several weeks. During all this time I was riding the bike on a daily basis so all pieces were getting used regularly and frequently.

Theoretically, the MTS MC and the Oberon SC should be a match pairing than the stock MC and the Oberon. According to a chart someone posted on this forum the sstock MC and Oberon combo should have had a 58% increase in travel but a 30-some% decrease in effort needed. Somewhere during this greater period I changed to ASV adjustable levers and had adjusted the clutch for a longer pull. Other than that, everything worked fine.
 

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Discussion Starter · #26 ·
my mityvac came with a selection of black rubber adaptors for bleed nipples , with the right one selected it's a tight fit on the nipple as other end fits tight into opaque tube from bleeder .
A question is the clutch hose an orignal rubber hose as it maybe well past its use by date. I had a suzuki gsxr 1100 with similar bleeding issues . Orignal hose was a long run and it was expanding. New Hel braded line fixed the issue .
Clutch hose is a steel braided line. Although it is most likely original.
 

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Discussion Starter · #27 ·
As one other member said are the master cylinder seals in good shape . if all else fails you may have to disassemble clutch master cylinder. if clutch hose is not past its use by date.
The MTS MC came off a low mileage 2014 MTS Pike's Peak a few yrs ago. The MC hasn't seen much use so one can think the seals are in reasonable condition.

The other thing is I have 2 stock ST MCs, 1 stock MTS MC, 1 stock SC and 1 stock Oberon SC. The only combo I haven't tried yet (and I will later today or tomorrow) is the MTS MC and the stock SC. But basically, I've been having this clutch lever losing pressure after a short time with every combo of MC/SC. That said, it seems to be a little better with the MTS MC and Oberon SC than the other combos.
 

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Discussion Starter · #29 ·
View attachment 1052532 That is the kind of sludge in a brake caliper from a K6 Suzuki 1000.
Bleeding does not get rid of it as bleed nipple is at the highest point.m
I used to rebuild Porsche Brembo calipers. Have likely rebuild at least 100 sets. I have NEVER seen anything like what you are showing before. Seriously ugly! I hope my SC doesn't look like that.
 

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Discussion Starter · #31 ·
BMW S1000RR manual states for bleeding front calipers , is to first remove caliper with brake hose connected , then clean pistions of road grime . Then push pistions in and place a wooden block to keep the pistions in and bleed brake capiler. Have you tried bleeding nipple on MC with mityvac after bleeding SC
I will try this also. I have several actions/techniques to try it seems. I WILL get this operating correctly.

FWIW when I went out this morning, after having sat overnight after riding, the clutch lever felt GREAT!
 

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Discussion Starter · #34 ·
Assuming that you're using fresh crush washers and good, fresh fluid with each install, then the line would be most suspect.

FWIW, the MTS masters have had mixed reviews and many folks, including myself, have had issues with them needing constant bleeding. Conversely, I've had zero issues with the (still!) original 23-year old master and slave on my ST2. Though I did replace the line when I upgraded the brake lines ('99 ST's were all rubber lines).

Given the issues I've had with my MTS clutch and given the unknown condition of the used MTS master, I'd be inclined to go with a new OEM ST assembly.

Regardless, I feel your pain, as my MTS clutch has been giving me grief for a while now.
Up until 2-3 months ago when this clutch started going sideways, I was having no issues with the stock MC and either the OE or Oberon SC. Then, as I've laid out in detail in previous replies, things started getting weird.

Theoretically, the MTS MC and the Oberon SC should be a reasonably good combo with reduced lever pressure necessary.
 

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Discussion Starter · #35 ·
Well, I finally got the system bled properly and the MTS MC and Oberon SC seem to work well together.

What did I do to accomplish this after all the hiccups? First I started replacing the crush washers with new ones. Don't know if that was the critical issue or now but a VERY knowledgeable friend said he has experienced issues with used crush washers previously. Then I proceeded by bleeding again from the SC first by pumping the lever several times, holding the lever closed then cracking the bleed screw, watching the fluid for bubbles (just a couple VERY tiny ones) then closing the bleed screw and finally releasing the lever. Then I repeated this process several times until there was and had been no air bubbles. Then I went through exactly the same process with the bleed screw on the top of the MC. At first attempt, there was quite a bit of air which surprised me as I had bled this screw previously. Nonetheless, repeated the process several times, each time pumping the lever a lot to build pressure. I finally felt everything was bled properly when after closing the respective bleed screws and releasing the lever, immediate re-application of the lever demonstrated consistent back pressure.

Took bike for a short ride where I did A LOT of operating clutch/shifting and the clutch lever remained consistent in feel and operation. Getting ready to take it for a longer ride today. At this point I expect all to operate correctly.

Next project is to convert the brake MC from the stock axial type to the 2014 MTS radial style.

FWIW, although I have heard many say the radial style MCs give better control and feel over the axial style based on just the clutch MC change over I personally don't buy it. I really am not noticing any real difference between the feel/control of the axial vs the radial. I do like the race-style reservoir, specifically the diaphragm design much more than the diaphragm in the "coffin" style stock ST reservoir. The stock design is terrible IMHO. I think that may have been the root of previous issues with the stock MC.
 

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Discussion Starter · #37 ·
You don’t have a bad ‘shake the bottle’ habit do you?
Like when you pick up a milk bottle, walk to the table and unconsciously shake it.

that’s bad for brake fluid.. but seen people do it.
No I don't have a shake the bottle habit. (Not that such an action would actually have any effect on brake fluid)

Shaking brake fluid will not add additional air to it. That is an old mechanics tale. Brake fluid does have microscopic air bubbles in it (2-5% SAE J1705 Appendix A if I recall correctly) but you can't add additional air by shaking the bottle (My day job is in brake fluid. Web site is torquebrakefluid.com)
 
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