'78 350SL 4speed manual, clutch questions

[jaybutter]

I have a gray market '78 350SL 4 speed manual and have a few questions about the hydraulic actuation of the clutch. The problem is that the clutch disengagement requires pushing the pedal down to the bottom of its stroke. When releasing the clutch pedal it begins engagement just 1/2" up from the bottom of available pedal travel. Air in the hydraulic system is my first concern. I have tried to do bottom up bleeding, due to the vertical master cylinder, with only limited success. I was most successful using oil can filled with brake fluid and pumping it into the slave cylinder via the bleed port. For me more fluid leaks out around the bleed screw threads than goes into the system. This was much worse when I tried using the brake system as specified in the manual. Working by myself without a car lift I am having trouble. I am hoping that someone has some experience with a SL clutch setup and can provide some direction and answer some questions.

1. My pedal movement is light to the touch as compared to my 220D which uses the same master cylinder. Is this due to the over-center spring assist on the SL?

2. At what point in the pedal travel does your SL clutch engage/disengage?

3. What bleeding method have you had the most success using?

4. Is my pedal so low due to some other problem besides bleeding? Other than being low, the clutch works well, has good grab modulation, no slippage etc.

5. What is the best way to determine if you have air in the system, other than bleeding?

Thanks for your help in advance!

[Frank Reiner]

Your problem may indeed be due in part to air in the hydraulic system, however it can also be due to misadjustment of the eccentric bolt that connects the clutch pedal to the push rod of the clutch master cylinder.
Access to the eccentric bolt requires removal of the cover panel beneath the steering column. If the plastic end of the pushrod has not split or is otherwise damaged by wear, there should be enough adjustment available in the eccentric to completely remove any free play in the pedal. If you are able to, remove the free play, then back off until there is approx. 1/4 inch of free play at the pedal, and lock the adjustment.

A caution is in order here: the push rods are available in 2, perhaps 3 different lengths. If one has been installed that is too short, it will not be possible to properly adjust the free play.

[jaybutter]

That is great information and pointers. This weekend I found the biggest part of the low engage/disengage pedal problem! At some point a previous owner or their mechanic replaced the slave cylinder with the wrong part number. The inside diameter of the wrong, existing one, is 30 mm and the correct one is 26 mm. So given a smaller diameter with the same amount of fluid pushed in by the master cylinder means that the slave piston moves farther, about 25 percent in this case! So now the engage/disengage point is about mid travel of the pedal and the feel of it is much better.

The correct part, acording to the EPC, is used only on 280 and 350 SLs and two W116 engine setups, I would guess 280 and 350 as well. The only source for the part that I could find was MBUSA and it was a bit pricey. I am having a little trouble wrapping my head around a gas W116 with a manual transmission though!