I've been looking for insight or a diagram.
Bench bleeding a brake master cylinder is all I can find.

I'm about to replace my clutch slave and will be attempting this process:

Bleeding the clutch slave cyl. without tears!

Do I connect a temporary reservoir to the slave and pump it repeatedly until it's full of fluid? Do I need the bleed nipple in circuit?


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1981 240D 4sp manual. Ivory White.

You don't need to "bench bleed" a cylinder with no reservoir if you just bleed it from the RF brake caliper. If you're worried about all that "dirty fluid" from the caliper, get rid of it before you start.

Understood, but not my intention.

I'd like to do exactly as outlined in that post.

My question, is whether I simply pump the piston and fill from the top, or add the bleed nipple in circuit.

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1981 240D 4sp manual. Ivory White.

Shern:
Yes, arrange a temp reservoir connected to the pipe connection port on the slave cylinder (SC). With the SC tilted so the port is high compress the pushrod into the SC, and connect the reservoir. Allow the pushrod to extend so that the fluid follows and fills the SC. Slowly compress the pushrod to force the fluid and any air back into the reservoir. Allow the pushrod to extend again, and you have a full SC ready for installation.

Thank you Frank!

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1981 240D 4sp manual. Ivory White.

Bleeding the clutch hydralic circuit. Seems on average made difficult by the clutch master cylinders internal rubber not clearing the fill port very well.

Removal of the clutch pedal pushrod should make it far easier. Possibly the way it was handled on the production line.

The struggles some members have had seemed to be really painful over the years I have noticed. It almost made me suspect that the fill port being clear was not an absolute requirement on that system. After it was bled. I have not replaced any component myself in that system.

I have absolutly no intent to go through what some members have either. If I ever have to. If i can blow easily through either port on the master clutch cylinder It gets installled without the pushrod. Fill the system from the bottom. Then install the pushrod.

Actually by design that system may not actually need bled as such. Rather just filled. Just from the bottom though.

A brake master cylinder has to allow fluid addition as the brake pads wear. This is not a requirment in the clutch throwout system. Once the working system is full it will never need more fluid unless a leak develops.

So why is it really connected to the brake master cylinder? When filling the surplus fluid would flow out the top of the master and eat the paint or present a liquid problem.Now if otherwise. Each pushrod is custom fitted to each installation there might be no problem. To expect that in series production with the nominal varients present is unrealistic.

So attempting a refill of the clutch hydralic system with the pushrod in is kind of asking for an issue. The master clutch cylinders rubber needs to be fully retracted.

If I am right another benefit is the brake fluid collecting moisture over time. The moisture contaminate fluid can not enter the clutch circuit, it is a sealed system if the pushrod is a little longer than it might be. Just some guessing by me.

Dont worry about the nipple on the side for the clutch hydraulics.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660

If the nipple is nor in fact at the top of the slave cylinder. By design it is to bled by displacement by the fluid being charged into the system. Any air will stay ahead of the fluid due to the much higher Denstity of the fluid. I cannot visualise and place in that simple system that could hold air in the presence of fluid. I see a simple reason that the master cylinder appears to be a resupply system.Where functionally it is anything but on any master clutch cylinder with a pushrod installed that is long enough to make the piston cover the top port.

Again I refer to the production line for the car originally. There is no time for the factory installer of the clutch fluid to fool around as so many members have landed up doing. If the input port is not covered this should be the easiest job pretty much on the car in some ways.

You bleed to get air out. Where it should be automatically displaced in that setup. You also really cannot bleed a closed system. Especially if there is no provision to do so. That is definatly the case with the clutch hydralic setup. There is just nowhere to hold air as the fluid level rises on bottom fiiling.. There also may be a slight design issue with the master cylinder. Or in an old master a swollen rubber cup in there.

Bear with me that I never before considered any issues. Of why so many people where having issues filling that very simple system with fluid.If one wants to pre prime the slave cylinder. You would press the piston in and hold it. Add fluid to it. Then as you held the slave with the opening as high as practical. Probably required to fill it anyways. As the piston came back it would push the fluid out with the air leaving first as it is on top of the fluid.

Stoking it or pumping it will not get any air out. Unless you are adding fluid during the process. when I seem to make no sense. I have no issue with it being discussed. Usually by the time it is hashed over. People have gained an understanding or insite they may not have aquired or considered otherwise.

We have some questionable termanology in play for the clutch hydralic set up as well. The bleeder on the slave cylinder is not the bleeder. It is the filling device. The system bleeder must be at the highest point in this system where air will arrive at. That is the exit port of the master. There is nowhere else it can get trapped. Other than just below that exit port. You are going to be in a world of suffering if that exit port is sealed. Sometimes things are so simple there seems no need to understand them.

Also be kind as my eighty year old brain in a few months. Is not what it once was perhaps. I do not profess to be right. I do like to make people consider things. I can be wrong and take no offence in it being mentioned. People that have no tollerance tend to have locked minds. My way or the highway has no appeal to me. Perhaps the simplest test to establish if that exit point on the clutch master cylinder is open. If the line to the slave cylinder is not connected or the bleeder is open. Fluid should flow by gravity out that line. In enough quantity to indicate the master cylinder is involved in providing the supply. If it does not pass that test. Pull the clutch master cylinders push rod and test again. Beats tearing you hair out trying to make it work. Unless you are bald.
If that fill port was not closing off. You could let the clutch system just gravity fill. Any air would just find its way back to the master brake cylinder. Air would be slow moving upward though. I also think by simply opening the bleeder on the slave it would also simply gravity fill with no air in that system.

The more I think about this. Mercedes may have by design intended the exit or entrance point to the master clutch cylinder quantified by the application being used. To be closed off when the pushrod is installed. The why would be simple. Brake fluid is so hydroscopic. If the primary fluid in the master gets into the clutch hydralic system. We should be seeing more failure rates than we are. The pushrod may just be longer than required by either design or accident. I almost dislike it when an issue stimulates me.

Strange, I have NEVER had to bench bleed any clutch slave. They are small and the fluid path is simple. I bench bleed the master and perhaps drain the old fluid out of the line but the clutch slave hasn't been a problem.

I do change or rebuild the slave whenever the master is changed because the slave has the same age on it as the master and fails soon after the master has been changed. May as well do the entire job which includes master, slave and hose (if any sign of cracking).

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

LOL what's happened here?

Did you guys read what I linked? It's a good post by Beagle.

I've bled the clutch using both RH brake caliper and the oil can method.
Both great, never had any trouble with either.

This time, I simply wanted to try a different method, which worked very nicely, and had the added benefit of requiring the least amount of effort.


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1981 240D 4sp manual. Ivory White.

This is analogous to Occam's Razor; the solution that requires the least effort, and the description that requires the least words is the correct one.
The others however, do keep Pampers in business with the excessive oral diarrhea.