Clutch Master/Slave cylinder--Piston Movement

[Atazman]

Gents......

Can someone tell me whether the springs inside the clutch master and slave cylinders have enough pressure to force the pistons out against their stops?  I'm talking about a situation where you have the cylinders on the bench, and no hydraulic lines are attached.  Should the piston/seals move freely enough to where the internal springs force the pistons outward?

Although the seals in both of my cylinders are holding (no leaks), when I manually compress the pistons, they will not return because of the opposing spring pressure.  I'm thinking this is not normal and it may be the reason my clutch pedal is hanging up at about the midpoint of its travel. 

I have read many posts about how difficult it is to get these cylinders bled.  I would think it is practically impossible to bleed them if the clutch pedal is not moving the master cylinder piston back and forth.

Appreciate your response.

[JimVillers]

Don .... The springs in the cylinders are not the spring that returns the clutch pedal.   You are either missing the clutch return spring (most likely) or there is corrosion in the pivot or push rod that hangs up the pedal.  Try moving it with your hand and look for binding or the missing spring.

[Atazman]

Thanks, Jim, for responding. 

My clutch return springs are there, Jim.  Both the big one under the dash is there, and the little one that connects from the slave cylinder to the fork working the release bearing.

I'm just guessing that the only purpose for the springs inside the master and slave cylinders is to enable bleeding those cylinders.  Jim, have you ever had either the master or the slave cylinder on your work bench, and pushed in the pistons to see if they return outward under the pressure of those little springs?  My thought is that they should keep the piston up against the retainer in the cylinder.  So....if you push the pistons in, the springs should push them right back out.  Correct?

Your statement brought up something else I'm wondering about.  With my master cylinder out sitting on my work bench, I push the clutch pedal down toward the floor.  It actually snaps against the floor, and stays there until I pull it up by hand.  Is this normal?  I'm thinking this situation is probably normal because there is no force to counter that huge spring under the dash.

However, once the hydraulics are installed and the system is fully bled, I can see where the main force causing the clutch pedal to return to its "out" position is the pressure plate springs acting against the release bearing and the clutch fork.  Those little springs inside the cylinders would not do much at this point.

Am I close to understanding how this hydraulic system works?

[Benz Dr.]

Yeah, you kind of have it. There's a small spring under the piston inside of the master and it will push ther piston back up to the stop near the top of the cylinder. If it won't move back up on its own then it's likely binding or stuck.

The pedal will go to the floor if there's no fluid in the system or the free play in the linkage has too much excess travel. The push pin at the slave cylinder should be just a bit loose once everything is bled and adjusted properly. I remove the spring near the slave cylinder to check this. If you need to adjust the push pin keep turning it out until you have about 20 mm of free play at the clutch pedal inside of the car. Have someone move the pedal for you until it feels right.

On some 280SL's there is no adjustment at the slave cylinder it's just a rod sticking out. I think there may be a small amount of adjustment on the push rod for the master; check where it is before you remove anything so it will go back in roughly the same place during reassembly. As long as the check valve in the master is working the clutch system will be fairly easy to bleed.

[Atazman]

Thank you, Dr. Benz (or Benz Dr. ).  Not only have you helped me with my Benz, but you also provided mental therapy. 

I have a new master cylinder on order since the piston in my old one seems to be too snug in the cylinder.  Well.....I also did not like the looks of some slight corrosion on the bore and my dust cover had deteriorated.  I never investigated wheather just a "seal kit" is available for these master cylinders and slave cylinders.

Thanks much!!

[JimVillers]

Don ... It sounds like you have become the "Clutch Expert".

Next is the fun task, bleeding the system after it is installed.  After much frustration, I read about the brake system back bleed procedure (using the brake system to push fluid from the slave back to the clutch reservoir, connect a hose from the right caliper bleed screw to the slave bleed screw). 

[Atazman]

No, Jim..... I'm far from an expert.  I've come to the conclusion that "all I know is what someone else has told me".  Unfortuantely, that applies to a lot more than just clutch hydraulics.  And thanks for the clarification on the bleeding technique.  I just went through my front brakes, so that system has fresh hydraulic fluid to squirt into the clutch system.

My new master is supposed to arrive next Monday (6/8); I'm anxious to see what happens.  As always, I appreciate your help.