Theory on why clutch bleed can be difficult - PeachParts Mercedes-Benz Forum

gastropodus · #1 05-14-2012, 04:29 PM

I changed out all the hydraulic components on my 240D clutch this weekend, and spent a fruitless afternoon trying to bleed the system. I'm not the first to have this problem, judging from the number of frustrated postings!

I'm familiar with the oil can video, and I was attempting to do a bottom-up bleed with something similar: what I did was drill holes in the lid of a peanut butter jar, mount some barbed fittings in the lid using epoxy, and then hook this up to an air pressure source (a dry garden sprayer, if you're curious). The idea is similar to the oil can: the jar was filled half full of brake fluid, and there was a pickup hose going through the lid such that as air pressure was introduced to the jar it would push fluid out a hose. This hose was connected to the clutch bleed nipple. So, pumping the air pump pushed air into jar, air in jar pushed fluid into clutch system. Or so goes the theory.

In practice, I just couldn't get fluid to go into the system at all! It leaked from the slave cylinder nipple, even after I introduced a hose clamp onto the hose to hold it more securely onto the nipple. It was as if there was something blocking the circuit that should be free going all the up to the reservoir when the clutch pedal is not pressed. I checked the eccentric shaft at the top of the clutch master cylinder; it was set to the position that should push the rod into the cylinder the least.

Finally today it hit me: I recalled that when I bolted the master cylinder into place, I had to force the rod down a little bit to get the the bolt at the end of the rod to line up. I'm guessing that the little bit of movement on the rod closes off the inlet port on the master cylinder so that now the master cylinder barrel and the slave cylinder barrel are a closed system - no place for air or fluid to go!

So, the master cylinder came with two rod lengths. I chose the rod length that I thought most closely matched the one I took out, but I wonder if that was the longer of the two rods... I probably need to try the other, shorter rod length.

Kurt

aaa · #2 05-14-2012, 06:27 PM

I would love to hear about the end result of trying a shorter rod.

gastropodus · #3 05-19-2012, 07:34 PM

Ok, this is getting ridiculous... like everybody who has ever tried to bleed a W123 clutch, I'm still having problems. I've got bottom end pressure bleeder setup made from a garden sprayer and a peanut butter jar half-filled with brake fluid. I literally can't force fluid into the system. First I put the shorter master cylinder push rod on in place of the longer one I had originally. When that didn't work I took the rid off completely; still no joy!

In desperation I undid the pressure line connection at the master cylinder - now there is just the slave cylinder and the hard line, and fluid still won't go into it! Is it possible I have a bad slave cylinder?!?

Kurt

Stevo · #4 05-19-2012, 07:52 PM

Have you made sure the slave bleed nipple is not plugged? Since it is in the lowest part of the system crud can collect there.

layback40 · #5 05-19-2012, 08:01 PM

I cant understand why you guys have so much trouble. IIRC i first bled a clutch when I was 12 or 13. An old beater on the farm.
Its important to get the MC full of fluid first.
Then with a long piece of clear tube on the slave nipple, that goes into a jar with some fluid in it, loosen the nipple.
Push the peddle to the floor gently & before you release the peddle, have some one tighten the nipple.
Repeat 4 or 5 times.
You will be able to see how much air is still in the system by looking for bubbles in the clear tube.
Once there are little or no bubbles its done.
Often there will still be a little air in the system that will work its way out after a few days of use.

sokoloff · #6 05-19-2012, 08:07 PM

I've always bled the clutch on my 240D from below as a last step when I pressure bleed the brakes.

aaa · #7 05-19-2012, 08:46 PM

I think I'll try this externally next time, I have all the pieces out for a 5 speed swap. I could try the slave alone, to see if something wrong with the newer bleed screws or something, then work my way up adding components to the system, until I find whatever is blocking it.

Stevo · #8 05-19-2012, 08:48 PM

Quote:

Originally Posted by sokoloff

I've always bled the clutch on my 240D from below as a last step when I pressure bleed the brakes.

When you do as you describe you are flushing the clutch system, not "bleeding" air out. That is if I understand, you have the pressure bleeder pressurizing the brake reservoir then open the clutch bleeder.

Stevo · #9 05-19-2012, 09:03 PM

Quote:

Originally Posted by layback40

I cant understand why you guys have so much trouble. IIRC i first bled a clutch when I was 12 or 13. An old beater on the farm.
Its important to get the MC full of fluid first.
Then with a long piece of clear tube on the slave nipple, that goes into a jar with some fluid in it, loosen the nipple.
Push the peddle to the floor gently & before you release the peddle, have some one tighten the nipple.
Repeat 4 or 5 times.
You will be able to see how much air is still in the system by looking for bubbles in the clear tube.
Once there are little or no bubbles its done.
Often there will still be a little air in the system that will work its way out after a few days of use.

Are you saying you bleed the clutch just like the brakes? You haven't bleed an MB system down on the farm, it doesn't work like that. You must go from the bottom up to get the air out.

layback40 · #10 05-19-2012, 11:54 PM

Quote:

Originally Posted by Stevo

Are you saying you bleed the clutch just like the brakes? You haven't bleed an MB system down on the farm, it doesn't work like that. You must go from the bottom up to get the air out.

You can go from the bottom up with brakes as well if you wish.
Unless a MB clutch has some unique system & looking at the FSM it dont. You can bleed as you please.

w123love · #11 05-20-2012, 12:18 AM

Quote:

Originally Posted by layback40

You can go from the bottom up with brakes as well if you wish.
Unless a MB clutch has some unique system & looking at the FSM it dont. You can bleed as you please.

I must concur with the bottom up method...I tried for days to do bleed normally and also with a vacuum on the bottom. It just isn’t possible in these MB systems to get it all out via a normal bleeding process. Though it was possible to build clutch pressure, I needed to bleed from the bottom up [pressured] to get ALL the bubbles out.

vox_incognita · #12 05-20-2012, 08:11 AM

Alas, bleeding is difficult...for that reason many people just choose to neglect the system. Have a look at this(last page describes the procedure):

http://www.vintagetest.net/chassis/enis73/V2I736040.pdf

Stevo · #13 05-20-2012, 10:21 AM

Quote:

Originally Posted by layback40

You can go from the bottom up with brakes as well if you wish.
Unless a MB clutch has some unique system & looking at the FSM it dont. You can bleed as you please.

Maybe is does work from the top down where you are, I've heard hurricanes rotate in the "other" direction as does water going down the drain plus you folks are upside down as proven by looking at a map of the earth. Therefor "from the bottom up" to us would be from the top down to you.

There, problem solved, we are both right.

sokoloff · #14 05-20-2012, 11:23 AM

Quote:

Originally Posted by Stevo

When you do as you describe you are flushing the clutch system, not "bleeding" air out. That is if I understand, you have the pressure bleeder pressurizing the brake reservoir then open the clutch bleeder.

Correct, the pressure bleeder is on the reservoir just like it is for the brakes. I have seen/felt no downside to my doing it this way. Clutch still feels fine afterwards and the clutch bleeder screw always produces the dirtiest fluid of them all. I don't think I would want to push that dirty fluid back up into the reservoir? Does not the pressure force out any air if there was some just as with the brakes?

charmalu · #15 05-20-2012, 11:46 AM

In this thread Beagle describes a different approach:

Bleeding the clutch slave cyl. without tears!

Another by Beagle:
Clutch Bleeding ,the best technique Ive seen yet.

You Tuve Video:
BMW, VW, Porsche Clutch Bleeding - YouTube

This is a single post from 2003, where Autozen describes the oil can method:
Pilot Bearing

Single post:
Clutch Bleeding ,the best technique Ive seen yet.

This is from Numbercruncher in the UK:
Clutch Bleeding ,the best technique Ive seen yet.

Another method by Dirtkurt:
Clutch Bleeding ,the best technique Ive seen yet.

I remember seeing these brake fluid pumps when I was a kid, think every gas station had one.

OK, last one.

This is from the famous Jimmyl, a good pictorial:
Replace clutch slave cylinder and bleed clutch

When doing the Factory method as outlined in the FSM, going from the R/F brake caliper to the Slave cylinder, to first power flush the brake system and the clutch to blow out any and all contaminates.

Maybe first use your wife`s Turkey baseter to suck out as much of the old fluid first.

When I swapped the 4-spd into our 85 300D, I power flushed the clutch with my Motive. I blew enouth fluid through it, there couldn`t possibly be any air in the system. still no clutch.

Used the FSM method, still nothing.

Used the oil can method, still nothing.

Even tried to gravity bleed it, but nothing would drip out.

I read where Gavin Leslie did this, and all was good.

Finally out of frustration, had my wife pump the clutch peddle, hold, open slave bleeder a few times, and it worked. what the....

although the peddle was down lower that I would have liked it, it worked and if it works, don`t further mess with it.

Since then I removed the transmission etc..... and am back to the bleeding problem. that was 19 months ago and the car has sat since then. but since the 240 drives..................... do need to flush the 240, so need to get at least one working back up car.

Charlie