Clutch bleeding: science question
 

My son and I are trying to bleed the clutch ('88 924S) the traditional way (#2 of Clutch Slave Cylinder Replacement) but what I don't understand is when I look at the slave cylinder->clutch hydraulic lines->clutch master cylinder->brake reservoir alignment it is all "up hill" without any place to stop air in the system from simply flowing/bubbling up to and into the reservoir given a little time. So my curiosity wonders why it doesn't? Any scientists out there? Along those lines it seems backward to bleed the air down through the slave when everyone knows air in a liquid (brake fluid) wants to go up not down.
We have quit for the day, but the last several "push to the floor and hold" of the clutch pedal just pushed out clear "air-less" fluid yet the pedal does not return to the top on it's own. There must be some air somewhere, maybe a bubble in the master cylinder.
FYI the project was to refresh the fluid while I also replaced the hi pressure (23 years old) and lo (16 years old) rubber clutch lines.
I know "get a Motive pressure bleeder". But I don't have one and want to just do it this one time. Maybe I'll get one for later but for now I just want to get back on the road.

YouTube - BMW, VW, Porsche Clutch Bleeding

You've got the right idea from a science standpoint. My method is a bit more complicated (and kind of ridiculous), yet pretty effective. I would fill the master and slave cylinders with fluid while off the car and worked the air out of it that way. Then I just rigged up a reservoir with a turkey baster and some hose, assembled the whole system, and just bled the assembled system out of the car so I could orient the parts in such a way to help the trapped air escape. You can feel right away when it's working right. With enough fluid in the baster, you can use the slave to pump the air up through the master and into the "reservoir" and then slowly retract to suck in fresh fluid after the bubbles float to the top. Repeat a bunch of times until you're satisfied that there's no air left in the system This would probably also work on the car with just the slave dismounted and the line indexed so that the slave inlet is the higherst point. Then I'd fanagle the whole thing into place, loosen the line connections just enough to index everything properly during bolt up, and then do a final bleeding just to make sure it's all good. If you are just replacing the slave and the master hasn't been run dry, just fill the slave half way up with fluid, work the piston to release any air in the seal, fill the rest of the way up, connect the line, and bleed normally. If the master has gotten air in it and you don't want to have to take the whole thing out, your best bet is the motive bleeder or the bottom up method in the vid, but I have tried neither and can't comment on them.

So much trouble. Motive Power Bleeder!

+1 - best investment I've made in a long time. Really simplfies bleeding.

Thanks everyone for the advice
 

Thanks for the You Tube reference. I think I will try that. And yes a power bleeder is on my Christmas list, I just have to be nice till then.

Also I just noticed on this project that the clutch master cylinder nipple for the reservoir fluid is a cheap plastic nipple i.e. I broke it getting the braided hose off; without much effort I might add. Luckily (whew!) I had an old clutch master in the bone pile and it had a fine nipple that I could pirate. The plastic nipple pops into a rubber grommet and seems to be holding fluid.

which motive power bleeder do you guys get? and where from?

http://store.motiveproducts.com/single-adapter-bleeders-c16.aspx

My take on the science question...and why the "traditional" (out the bottom) bleeding method works at all, is that the combination of brake fluid viscosity, plus the relatively small diameter of the hydraulic tubing, means that bubbles in the system move uphill much more slowly than the fluid is being forced to move in the opposite (downward) direction by applying pedal (or other) pressure from above.

Still, I've got my engine out....and the slave is just hanging there by its tube - so I'm very tempted to just hold the attached slave up above the level of the reservoir, open the bleeder, and have someone pump the clutch to get the bubbles out. If this is done gently, I don't see a problem with having the plunger pop out of the end of the slave, as it has a retaining ring.

MaD924

This one: European Bleeder 0100 -- Motive Products Online Store

I actually taught my brother something automotive for a change. Information/technique exchange mostly runs the other way.

Pelican.
Pelican Parts.com - Bleeder Bottles

It sounds like father and son may have a couple more issues in addition to the broken nipple.

Suggestions.
Clarks suggests lifting the back of the car higher than the front. Block the front wheels, jack up the rear of the car and put a couple of 2x4s on top of the rear jack stands. You will need to do this to get the air out of the rear brake calipers anyway.

Check your rubber clutch fluid line between the clutch MC and slave. If it feels soft and spongy, replace.

Look under the dash at the cluch MC rubber boot. Do you see any drips of fluid? Pop the rubber boot, look again. While you are at it, do the same for the brake MC boot. Any drips--------replace.

If you think the video HondaDustR posted is worth a try, You must loosen/disconnect the clutch MC from the clutch pedal under the dash. Get a 13mm and 8 mm wrench and make sure there is no pressure on the clutch MC piston at the adjustment shaft between the two. If there is any pressure on the clutch piston, you will spray fluid in your face with the oil can when you pump it via the slave. OH, another tip, clamp the hose tight on the oil can and slave. BTW----I have done this and it works.

Finally, the final adjustment to any clutch fluid flush is the "clutch pedal adjustment". The free movement of the pedal is 2 to 3 mm .

GL
John

Fyi.......once you get the pedal bled where it will return to the up position if there is a few inches of no pedal when you first press it cycle the pedal about 5 times then hold it to the floor......then let the pedal "snap" up by slipping your foot off the side of it, and immidiately pump it 5 times or so very fast, this will suck a bit more fluid into the system and get rid of the dead spot.....providing you got all the air out of it......

Just bleed the damned thing from the coupler up top where the flex line connects to the hard line going into the slave!

It takes about 20 seconds to do it this way, you get rid of the air going "uphill", and your problems are solved.

The only downside is that you get brake fluid on your bell housing, but not much if you put down some rags to catch it.

Cheers, Keith

There is one more way to bleed the clutch system that is easier than all the rest -

Bleed from the bottom up...Get a Oil Squirt Can, the larger the can on it the better, a 3'-0" length of clear plastic tubing, a turkey baster...Place the plastic tubing on the end of the oil squirt can and fill the tubing with fluid, then place the tubing on the Clutch Slave Cylinder Bleed Valve. With the Turkey baster make sure there is room in the Master Brake Cylinder for the fluid being pumped up into it. Pump the fluid up into the Master Reservoir with the attached Oil Squirt Can. Once you see that fluid is rising in the Master Reservoir, and no more air bubbles you are done - close off the Clutch Cylinder Bleed valve.

It also helps to have a different color of Fluid to know for sure that the system is complete.

Have pfun --

Christmas list? It's only $50. . .you may be messing with the wrong car.

Hehe, or the car budget is always spoken for by more critical necessities. That's usually why I don't get to buy all the extra tools I'd like to have. It's one of those things where it's not so much you can't afford it, but you just know that as soon as you do, some $200 project is going to come up and then your garage budget is out $250 and throw in $15 more to ship the thing. For some of us it's a matter of stopping the bleeding (no pun intended) and keeping her comfortably on the road rather than being good excuses to buy cool tools, but then there are a couple of those now and then as well. :cool:

I bought my motive bleeder almost 10 years ago......used it on my 911 many times, 911 is long gone.....
Used it on my 924s a few times, and have been using it like mad at the shop and it has really proven it's worth way more than once......
Its one of those tools that once you have it you'll wonder why you've waited so long to get.....you won't regret it, it turn brake/clutch bleeding into a 1 man job and you'll be done in less than 10 minutes....

Stop! You're making more budget priorities! :mad:

Just to add my $0.02 ... I rebuilt the clutch master cylinder three weekends ago, and had a heck of a time bleeding the circuit. I watched the Youtube video, jacked the back of the car way up, the works. Finally, I did two things that literally made it a 10-minute job.
First, I got ahold of a Motive bleeder and about 1.5 quarts of DOT3 fluid. Second, I found that I had to pump the clutch pedal while fluid was flowing out of the slave cylinder into the catch can. If I didn't, I'd get a nice firm clutch pedal, but by the next morning it would go halfway down and not come back up. But 4-5 pumps while the power bleeder was pushing fluid through, and it worked like a charm.
Oh, and for posterity -- if you're doing work on the clutch circuit, be sure to check that blue flex line going to the master cylinder! Mine was starting to crack at the ends. Got a local VW repair shop to cut me an 18-inch length of the same stuff for $5.00.

Surface tension?
 

Thanks to everyone for their input!
I suspect that the answer to my original question is just that the surface tension of the air to brake fluid to tubing walls interface is just too strong to allow the air to simply rise up and "bubble" out the top even though there appears to be an uninterupted upward bias in the whole circuit.

I believe the job is done, won't know for sure until I get the intake manifold back on and try driving, but we did it with pressure from the slave up using a mity-vac's pressure hooked up to it's little bottle. Basically power bleeding from the bottom up. Messy but it worked and expelled a bubble into the reservoir and I have a pedal that pops back up as it should and feels firm. Since we had "bled" the system from the top down the "traditional way" all the dirty fluid was expelled out two days ago. I also have new rubber on both the hi-pressure & reservoir to master. Examining the old rubber hoses they both look great though and it could have waited several more years. I am always torn between preventative maintenance, which I firmly believe in and "if it ain't broke, don't fix it" as I have been bitten too many times by messing with something that was working fine.
Just to make a long story longer, yesterday I bought one of those hand oiler's (Plews-Edelmann brand) like is shown on the You Tube video and spent ten minutes cleaning the chinese manufacturing crud that I didn't want in my system out of the can, hooked it up as in the video and started to pump only to have brake fluid sqirting out the top center of the lid where the outlet tube came out of the top--typical chinese "quality", they didn't seal it. So now I have to take that piece of junk back to the store.

'Tis a no-brainer, DustR. . .especially if you plan to flush your hydraulics every two years, as you're supposed to.

My Motive Bleeder is on its way to me right now. My wife expressed deep, heartfelt gratitude...

I'm sure you know why.