Bleedin' clutch slave...

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BLS
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Bleedin' clutch slave...

Post by BLS »

That title can be taken as a derogatory comment or a question. It's both.

New Wilwood master cyl, new line, used clutch slave cyl. The line is AN4 with one fitting at each end. Under pressure I see no leaks, etc. The slave is vertical with the bleeder at the same level as the entry line. I have rear of the car higher than the front by about 10 inches.

I've bled this thing until I'm blue in the face. Clear line connected to the bleeder, going up to a bleed bottle (one way valve). I've run 2.5 qts + through this thing, always keeping the reservoir near full. I've bled it using the bleed valve and the old fashioned way - wife pressing pedal, break the bleed screw, close the screw, release the pedal - repeat.

I continue to get air in the line as it exits the slave. Probably 5:1 ratio of fluid to air. I've NEVER had any trouble bleeding anything before. There is no fluid leaking past the seal of the slave.

Is it possible the slave seal holds the pressure and seals when pressing the pedal, but sucks air when released? That is about the only thing I can come up with. Any other thoughts appreciated. It just seems ridiculous. Oh I should add, the slave has been unused for 6 years. I took it apart during the rebuild and the seal looked OK. Maybe it's just hardened and holds when pressurized but not when vacuum?
Barry
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Re: Bleedin' clutch slave...

Post by FV80 »

Barry,
I doubt it's 'sucking air' back in through the pressure seal.. MUCH more likely is that is is just leaking a bit of air at the bleed screw causing you to SEE air but it's NOT coming from the slave.

Close off the bleed screw. Do you have good slave/clutch travel when the pedal is depressed? If you HOLD the pedal a few minutes does the slave hold it's position? When the pedal is release, does the slave return properly? If the answer to all is YES, then all is probably just fine. I see air in the bleed tube quite often when doing brakes. Some times I can "fix" it by repositioning the tube on the bleed screw .. sometimes I can't. I also had to cut the end off the bleed tube (~ 1/4") to get a better fit. The tubes get hard with age and don't seal like they did when new.

Steve, FV80
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Re: Bleedin' clutch slave...

Post by BLS »

Steve,

Checked the line attached to the bleed screw, it's new and tight so I don't think it is getting air from that. It's a bit hard for me to press the pedal from outside the car and observe the slave cyl travel, but it doesn't appear to move very much, maybe 1/2 to 3/4 inch, just takes up the free play I currently have. I changed the size of the master to get a short stroke, using a 7/8 diam master. So, I should be getting plenty of fluid movement with a little harder pedal. I think there is just still air in the darn thing. Pressing the pedal down and holding it, there are no leaks anywhere. It seems so simple...

I can't see how the seal could leak this way, sucking air in but not leaking fluid, but I can't come up with anything else. It does have a remote reservoir, but the connecting hose is tight, no leaks there. It's just baffling so I thought I would throw it out here as maybe someone else has seen this kind of problem.

Thanks for the advice. I'll keep trying.

Barry
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Re: Bleedin' clutch slave...

Post by FV80 »

Sounds like plenty of slave travel to me - assuming you are using a diaphram clutch and not the 3 fingered version (takes a LOT more travel than diaphram).

There is also a thread over on ApexSpeed, that you've probably already seen - people indicating that only Wilwood MC/s will work correctly under certain types of applications.

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Re: Bleedin' clutch slave...

Post by brian »

I posted some thoughts on bleeding a few days ago and if there is any area in the chamber above the bleed screw you may be trapping air in a pocket. You mentioned that the slave sits vertically and most cylinders I've seen have the bleed hole in the middle. Hold the slave in a horizontal position with the bleed screw in it's highest possible position relative to the slave and bleed again.
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.
BLS
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Re: Bleedin' clutch slave...

Post by BLS »

Steve, finally gave up last night. I'll get some help this evening and try again. I am using a wilwood but don't think it really matters.

Brian, this is one of those CNC slaves. The bleed screw is at the top. The problem is I keep getting air, visibly on every stroke. Now this is using my cheap bleed valve. Earlier I was doing the old fashioned way when I had my wife press the pedal while I broke the bleed screw. Maybe, with a new system I just still had air. Possibly, doing it with the cheap bleed valve air is sucking back in the loosened bleed screw. I'm going to do it the normal way again and see what happens. I've just never had a problem before... Frustrating.

Thanks much,
Barry
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Re: Bleedin' clutch slave...

Post by Dietmar »

Barry:

Following your posts and I have to ask: a used slave cylinder. How old, from where, and might this be causing the problem?

Dietmar
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Re: Bleedin' clutch slave...

Post by BLS »

Dietmar,

Looks like an almost new one that has been sitting 6 years. I took it apart and it looks fine, even the seal looked good. It's one of the CNC types. I found the specs online, the max stoke is 1.125".

I sent a reply to your email. I'm guessing the actual problem is in using the bleeder valve at the end of my clear tubing. It works fine, but I'm betting it sucks air around the threads of the bleed screw while it is cracked. I'll get some muscle on the pedal and finish the old fashioned way and I bet it will work. I hope.

Thanks for the help,
Barry
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Re: Bleedin' clutch slave...

Post by BLS »

Update:

Tried bleeding several more times, always seeing small air bubbles along with the fluid exiting the slave cylinder. Baffled is all I can say... :?: Until about 15 minutes ago :shock:

I had noticed a tiny drop of brake fluid on my new master cylinder, right on top, at one point. I mean small. Figured I just dripped on it from the brake fluid bottle when I was filling the reservoir. Since it is really bothering me, before I turn in for the evening, I go out to the garage one more time. Sitting there racking my brain over the problem, look down at the master, there is another tiny drop sitting right on the very top. Hmm, I'm certain I wiped that tiny drop off after last messing with it. Wipe it off. Pump the pedal. Let it sit a minute. Yep, the new Wilwood has a casting flaw. Even with all the pressure it is so small you would not notice while pumping. You have to pump it then wait before you can really see it. I'm guessing the darn thing sucks more than anything else as you would not see it until after it sat for awhile.

Just when I think I'm a complete idiot...

Thanks for the help,
Barry
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Re: Bleedin' clutch slave...

Post by SR Racing »

"casting flaw" ?

You mean at the pressure line boss? Are you using banjo or straight threaded fittings? If banjos, are you using aluminum or copper crush washers between the banjo bolt and banjo, etc ?

Unless it is a actual crack in the casting, it would seem this would be fixed by any of the above...
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Re: Bleedin' clutch slave...

Post by BLS »

Jim,

No, not at the fitting. It is actually in the body of the cylinder, right on the top, at what appears to be a casting "line' in the metal. It is a very small spot. Attached is a picture with the flaw circled in red. The second pic just shows a different view. The bolt in the end is just loosely threaded in place to stop bugs from crawling in the hole 8) . If you look at the front of the cylinder where the fitting is, then at the very top you can see in the picture the casting area I am talking about. About a half inch back from the front is where the tiny drop of fluid will come up. If I wipe it, then pump the pedal, with a light overhead, you can see a tiny reflection begin (reflecting off the fluid), then after it sits for a minute you will see the very small drop form, sitting right on top of the master. It is small enough that it just sits there, never large enough to even drip off the curved surface. It is definitely a flaw in the casting.

Edit: the pics were taken some time ago, before I ran the new lines, which is why you don't see the reservoir line or clutch line.

Regards,
Barry

Image
Image
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Re: Bleedin' clutch slave...

Post by jpetillo »

Barry, I didn't read this thread thoroughly, so I hope I'm not saying anything already covered or dismissed. Are you sure your bleed hose is tight? I had a similar problem a few months back. On my slave cylinder the bleed hose nipple is very small and my hose would not fit quite tightly enough. So when I bled it I kept seeing a constant stream of air bubbles. When I repositioned the hose, it bled clean. John
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Re: Bleedin' clutch slave...

Post by BLS »

John,

Thanks for the tip. I think Stevan may have mentioned that earlier. It is a tight fit however. I used some new clear tubing that fits it very tightly.

It's a weird problem. I should mention, the pedal is rock solid after bleeding. I just did not like seeing those bubbles. I have always been able to get brakes or a clutch to have zero bubbles in the line once bled properly. I really think it is the master cylinder casting flaw. Every time you release the pedal it would have to suck air in. It is a bit strange in that the flaw is so tiny that under pressure no fluid comes out unless you hold the pedal for a minute or so. Or, after it sits (with the back higher by about a foot - gravity) for several hours you will see the tiniest dot of fluid sitting on top at the flaw. About the size of a pinhead.

I have to travel out of the country for awhile tomorrow and have been to busy to investigate further. I have another master I can put on temporarily, just to test if it is the flaw causing the problem. I have taken the slave apart (3rd time?) and there is no fluid leaking at either seal.

I'll figure it out when I get back and post the result. Maybe it will help someone else someday.

Thanks again,
Barry
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Re: Bleedin' clutch slave...

Post by jpetillo »

BLS wrote:Every time you release the pedal it would have to suck air in. Barry
Yes, this definitely can happen. I had a front slave cylinder doing this. It was after the car sat for a year before I bought it. At the time, it was the last thing I expected - sucking air when the brake was released. I used an embarrassing amount of brake fluid along the way, and continued to have a steady stream of bubbles. I put in another wheel cylinder and the system bled clean right away. John
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Re: Bleedin' clutch slave...

Post by BLS »

John,

Sorry for the late reply, traveling out of the country.

It might be that I have more than one problem. The master definitely has the flaw. Might be another problem as well. My slave has been sitting for a while. The seals look good and they do not leak fluid under pressure. My original thought was the slave seal would work OK when pressurized and then let air suck by when the pressure is released. That still might be happening and your wheel cyl situation might have been similar. I'll get the master changed and if that is not it I'll put in a new slave or replace the seals.

Thanks Again,
Barry
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Re: Bleedin' clutch slave...

Post by sharplikestump »

Barry,
Interesting post. While I would certainly attend to the master (maybe gutting it and putting a dab of braze, if only for the test), I will venture a guess that as long as the slave sitting for that long hasn't generated a pit in the bore, a light hone and new seals will correct the problem. Be sure to use brake assembly lube.
I have seen where older seals can function normally under pressure, but lose their "wiper" ability on the return trip.
Mike P.
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Re: Bleedin' clutch slave...

Post by BLS »

Hello Mike,

I have returned from my trip and worked on the car a bit, mostly ignoring the clutch for the moment...

What I do know is the master leak is not the problem. A bit of fluid does appear over time but leaving it there for a visual check, it never gets sucked back in so that leak (while a problem to be fixed) is not the culprit. I'm pretty sure it is what you say, the slave seals under pressure but sucks air when released. I'll change the seals/hone or swap for a new slave and report back later.

Thanks for the advice,
Barry
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Re: Bleedin' clutch slave...

Post by BLS »

I had forgotten to update this just in case anyone does a search with similar problem.

I replaced the clutch slave cylinder with a new one, found a low price at Jegs for a Howe ($45) that looks almost the same as the CNC. That cured one problem, the air bubbles I kept seeing. It's not perfect but it is much better. After I got that fixed I discovered that my "engineering" (term used loosely here) to free up some length for my long legs and attempting to make some room for left foot braking in the Citation (steering column in the way) created an issue that meant it was very difficult to press the pedal far enough to get the clutch to disengage. Because I used a master with about twice the area of the original (to get a shorter stroke at the cost of higher foot force), the combination of the higher foot effort required and the pedal hanging up on a bolt near the end of the stroke made for difficult clutch work. I changed the pedal back to the original configuration and it works fine now with just a bit higher effort.

The master does have the casting flaw, which I have not fixed. It was never the actual problem even though it is an unusual "feature". I've had it so long now I'll just touch the spot with the tig rather than send it back.
Barry
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