Technical Can a slave cylinder suck in air but not leak fluid?

I am trying to bleed the clutch on my truck and I don't seem to be getting anywhere. I have run a ton of fluid through the system but still can't seem to get all the air out. The pedal will pump up hard, bleed it, get air in the fluid, close it release pedal and it goes right back to the floor on next push. I have done this over and over and over and the amount of air doesn't seem to be getting less so I am wondering if the cylinder could be sucking air in when it goes back out? I can't find any fluid leaks anywhere. Oh and the master is under the dash so way above the slave cylinder.

 

What kind of setup are you using?
Any pics?

Seen loose lines, loose reservoirs, bad cylinders etc suck air and not leak.

Is everything level?
Did you bench bleed the slave and master?
If the master or slave are mounted at an angle it can be hard to bleed the air out

If the slave is abouve the master it makes things tricky also

More info and we like pics here
Thanks

 

Is there a return spring on the clutch arm? You need to keep pressure on the slave cylinder piston as it retracts or it can suck air. Think of a drum brake wheel cylinder.

 

seems to me it could be either the master or the slave or both that are causing the problem. As mentioned in above post, is the spring return on the clutch arm keeping the piston in the slave 'loaded' on the return....or tube/hose connecting the slave and master partially block inhibiting fluid return.....the port in the M/C reservoir partially blocked...etc.

Ray

 

Yup, can suck air without leaking.
If you don't know when the component was assembled the cups/pistons may be sticking or cocked in the bore. I usually pull apart components to make sure any grease that may have used is not hardened up causing sticking.
Also are you sure you have the bleeder screw in the highest position?
Did you try bench bleeding either cylinder before install?

 

What kind of truck?

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It’s a hot rod 37 Chevy with plenty of modifications and a mixture of parts.

You sound like a parts counter guy at one of the chain auto parts places

 

No.... Info helps. Some clutch masters are mounted with the outlet downhill from the pushrod end and no amount of bleeding will get all of the air out unless the master is loosened enough to tilt it upward. Is yours mounted under the floor? Is the master lower than the slave?

Yup.... More info is required. Just go ask the parts guy.

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You do Need to bench bleed the clutch master just like any other.
Did you do that?

Clutches can be a bitch to bleed.
Air wants to go up, most clutch lines go damn near straight down.

 

I used the setup from a 89 camaro only way I could bleed the system was out side the car. Hanging everything from a ladder.

 

Bleeding a clutch system is a tricky job and it has often made a strong man cry. No insult intended but if you're doing it wrong 20 times, you're still going to have air in the system. It's usually best done with 2 people working in perfect synchrony, though those "speed bleeder" fittings can help with less than 2 peeps.

It could be a air leak, of course. If one person watches the uncovered MC reservoir while someone else pumps the pedal firmly, you may see some bubbles cycling to the top if it's sucking air.

I didn't write this and you didn't read it here, but if all is tight and you're dealing with a stubborn bubble, the "Redneck Bleed Dance" has been claimed to work. It's doing 20+ pedal strokes with no pause hoping to push said bubble to the top with just the force of the fluid flow........... But, naw, I would never do that. Nope.

 

Thanks guys. Everything is level, bleeder screw is straight up on the slave, I do have a return spring but I had some concerns about it being strong enough so I picked another up today but haven't had a chance to switch it out, I did "bench bleed" the master but I had to do it in the truck because it is a shit show getting it in and out and it would be impossible to do with any fluid in it so I am not fully confident on that and will try again. As mentioned in my first post the master is under the dash so way above the slave which is probably the issue. I will try again tomorrow and post back to see where I am at. I appreciate all the suggestions.

 

Yes it can......

 

You can try forcing fluid from the bleeder on the slave up to the master
Or if you can get one and are able to get it sealed to the top of the master a pressure bleeder is the cats meow


I know you said the master is under the dash
Something that has helped me in the past is bleeding the system in sections

Pump and crack the line at the master
No air left
Pump crack line at next joint/fitting
On down to the slave then last the bleeder

Hope something helps for ya

 

If you did the bleed over and over and still get the same amount of air, you letting new air in somewhere, somehow. Bones

 

I had the same problem with an MG. Pressure, gravity, lifting the car etc., nothing worked until I blocked the slave cylinder from opening during the bleed. A small amount of air was trapped and pushed into the slave every time I tried to bleed it. Soft second pedal every time. I used a small piece of wood between the rod and the frame. Look up bleeding MG clutch, youtube etc. It all depends on where the bleed valve is located, yours could be similar.

 

Pricey, but these work great,https://www.ebay.com/itm/Phoenix-Sy...752561&hash=item2354ae52ee:g:Mg0AAOSw-vlVoq8u

 

I went to look at your build thread and the photo bucket event has taken care of that. When I'm the only one available, I use a vacuum pump to bleed. I have the pricey one on the market but H.F. has a cheap one. Item 69328. From your description, you have to have an air leak or the pedal isn't returning all the way sometimes which would not allow the fill port to be open on the cylinder. Is there a chance that the slave doesn't return all the way to it's resting position that would not allow enough travel to move the throughout bearing? Because you've bled this several times with the same result, do you have someone who can look over what you have done to see if you're missing something? Just curious, is the slave a push or pull unit? Sorry I'm not more help. Without pics or being there, this is a guessing game based on past exp.

 

You might try putting a vacuum to the master cylinder reservoir. A mighty vac pump with the in ine bottle is what I use. You might have to push the pedal down with the vacuum on it. It always works for me in a pinch.

 

Well I am pretty sure I must be sucking air into the system somewhere as I get air out every single time I bleed and not just a few little bubbles. Is there a way to test the master and/or slave for air leaks?

 

When I'm faced with a seemingly impossible situation and head scratching material , I'll build it on the bench 100% and 9.5 out of 10 times the problem becomes apparent. There's still that 1/2 time though.

Why no pics or details of the parts you are using?
Hard to say, some clutch lines are goofy setups with o rings and roll pins.

 

No way to take them right now. Its a ECI clutch pedal set up with a Speedway pull style slave.

 

I use the Speedway pull slave. I didn't have air problems but I was wearing one out way too quick. I installed a swivel in the pull rod and that solved the problem. Is there a chance the angle of the rod is severe causing the piston seal to be ineffective? Another thought: if you didn't use 1/4" line, you might consider that. It allows for faster release of the clutch. List of places air can get into the system: the slave on retraction, a line but unlikely because it would also leak under pressure, the master cylinder being at an angle so the cylinder fill port was uncovered upon release, I never have seen seals installed in a master backwards but maybe there is a problem in the cylinder itself. Some time ago, someone had a master that wasn't assembled correctly and it was driven them nuts. If you made a cap for the master and connected a vacuum pump to it, it should pull down to the capacity of the pump and stay. If the vacuum leaked of rapidly you would have the beginning of where to look.

 

Where do you have a return spring? You shouldn't need one anywhere.

Without pics... We are still guessing.

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I'm not sure if this is the stuff or not?



Are you developing any pressure in the lines and slave?
If the internal pressure never exceeded atmospheric pressure it's certainly can suck air and not not leak.

Be sure the master rod is A, adjusted correctly and B returning fully to its home/resting position. That only works after its bench bled though ( to bleed it you need to work all the air out of it) I'm not sure how much of a PITA it is to get to the line at the master but you should be able to bleed it there and may need to. You can also cap it there and have a rock hard pedal. Did you run a hard line? Or did you run a braided line ?

 

So I took a bunch of stuff out so I could get the master in and out easier and bleed it on the bench. It doesn't seem to be sucking air in as once the air burps out continued pumping of the cylinder does not produce any more air. There is a swivel at the end on the clutch fork and the rod seems to be going in and out straight. The slave is moving when I push the pedal but the pedal just never seems to firm up unless I pump it and I am still continually getting air when I bleed. I was able to get some pictures although 31Vicky shows all the stuff in his post. This is looking pretty straight down as the bleeder is upright. My guess is the slave is sucking air. At this point I am thinking I am going to try and see if I can find a Wilwood slave in town or order one as I have heard a number of times now that the Speedway ones don't last so I don't want to go through all this trouble only to have to do it all over again.

 

The Speedway slaves have a short stroke that just barely worked on my Chevy set up. I tried a Howe and it was better. I tried not to use a Wilwood after reading MANY bad reviews, but it did have a longer stroke. Bought Wilwood, and it worked fine, but leaked after a few weeks. Summit replaced 2 for free, all leaked, then the third has been operating for more than a year. You have to be suspect, when in the ad for the product, they advise you to buy the rebuild kit with the product! When this one goes, I'll change my ways and convert to a push style from a mid 80's Chevy truck. The Wilwood has "O" rings as seals, instead of a cup. Poor design.

 

I'm wondering if your clamp on the slave is distorting the cylinder? If case you need a little more travel, remove the return spring from inside the slave. I've tried all the brands. I happy to buy the Speedway because it's the same as a CNC for less money.

 

I am a little concerned that maybe the stroke is a little too short and that might be some of the issue. I don't think the clamp should be distorting anything. I made it loose and then lined it with rubber so it would hold without applying any real pressure on it.

 

Looking at the picture; it appears that it is mounted at the rear and also clamped around the middle; how does it swing to match the arc of the throw out arm. Looks like there would be side stress on the rod during operation; which may let it suck air.

Also seems unusual for a slave to be mounted on the frame. Does it try to pull the engine back, too.