Clutch pedal travel not entirely transferred

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Hi, my car (Volvo S40) is equipped with an hydraulic clutch command. Today I wanted to check the pedal travel to compare with specs and found a surprising thing: the first 1cm of clutch pedal travel moves the master cylinder piston but this movement is not transferred to the slave cylinder. If I depress the clutch pedal by hand, I can feel there is almost no resistance for the first 1cm although the piston is pushed in the master cylinder. After that I can feel a constant pressure throughout the rest of the clutch travel. At first I thought it could be air in the system but I pressure bled several times and my Volvo dealer did it also but the same "slack" remains in the pedal. I have no particular problem with my gearbox but I'd like to know if this behaviour is normal or if I should pay attention and fix it. Most people will tell me that hydraulic clutch have no pedal adjustment but my car has and Volvo gives some specs for pedal height, travel and free play. They mention that free play must be adjusted without pushing the rod into the master cylinder so what I observe is not free play, it's movement of the master cylinder that is not transferred to the slave. Is there anything specific to know to bleed a master or a slave cylinder that cannot be achieve by a standard pressure bleeding??? Is it possible this 1cm slack is due to something else than air in the system??? At last, is this normal or should every bit of master cylinder movement be transferred to the slave cylinder??? Thanks
 

kilou

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Sorry, can't find the edit button anymore... I wanted to add that if air was present in the hydraulics, I should have a spongy feel throuout the whole travel. In my case, I have no resistance for the first 1cm, then a constant resistance for the rest of the travel. The clutch slave cylinder only starts to move when the constant resistance is reached. Would this mean that air is not the problem?
 
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Master may be right about the play. Are you sure the piston is moving, or just the rod? That is what your problem sounds like. Otherwise, I am not sure pressure bleeding, bleeds the master cylinder. You may need to disconnect the line from the master cylinder and bleed it. Connect another line to the port, and run it to the reservoir. Even a cork and tubing may work.
 

kilou

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Quote:
Master may be right about the play. Are you sure the piston is moving, or just the rod? That is what your problem sounds like.
Actually all I can see is the rod moving into the master cylinder. As I thought the rod and piston were connected, I assumed the piston was moving too. But you're probably right: only the rod is moving and not the piston. I've checked a little further in my Volvo tech documentation and they state that the free play (with rod NOT moving) should be 1-3mm but they mention that the free movement of the pedal should be 6-13mm!!! This is probably what I'm seeing and with about 10mm of "slack" (with rod pushed in the master cylinder) I may be within specs! But what would be this slack for? I understand free play is necessary to fully release the clutch but I cannot figure why up to 13mm of slack in the master cylinder would be necessary.
 
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The master cylinder push rod (the rod that is attached to the clutch pedal at the clevis pin) is not directly connected to the piston in the master cylinder; it just pushes on it. All you are feeling is the slack in the many components that transfer motion to the master cylinder piston. You can unlock the push rod at the clevis pin and turn it to lengthen it and take up the slack but really, as long as you can shift fine with no grinding and you can also feel that there is slack between when you start letting up on the clutch pedal and when it starts engaging, then all is good.
 
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Oh, they stipulate there be slack so the hydraulics aren't loaded per se, but member mechtech here who is an auto technician indicates it's perfectly fine to preload it a turn on the pushrod so there is no slack at all. I did that on my car.
 

kilou

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Since this slack corresponds to rod movement and not piston movement, the hydraulics should not be preloaded if the slack is removed. However you're right, until the box is shifting fine I'll leave it like this. I sometimes have grindings when upshifting in 2nd (when cold) or downshifting to 3rd but it's probably some worn synchros or a misaligned gearshift linkage. Needs to check that. Thanks for your help guys!
 
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