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MG MGB Technical - Clutch lever travel
|Is it possible for the clutch lever to travel too far? Master,slave and hose new. No movement at all on slave rod. Bled with rod all the way back, good pressure when helper depressed pedal no air coming out. Removed slave, rod came out about 1.5 inches. When I pushed it back in the pressure pushed the disk off the top of the master cylinder cap and pushed brake fluid out. The rod goes way back in the slave whereas the rod on the original slave was out about 3/4 inch. Rods are the same length. I tried moving the clutch lever by had and could not. Used a 16' adjustable wrench and found the first 1' of travel very hard then it moved quite easily. Guess if have two questions. How far should the rod protrude from the slave when forced back? Can the clutch lever come too far forward and if so how do I correct this?|
76 B DOT 5 silicone fluid.
|The slave cylinder position is self adjusting - as long as the piston and seal are contained within the bore (ie not popped out of the end) then the system will work. As such the protrusion of the rod isn't critical, though obviously if it's way too short the piston will pop out of the bore, or if it's way too long then it will be loading the clutch even when pushed right back into the cylinder.|
If I understand you correctly:
No evidence of air in the system after bleeding.
But the clutch arm doesn't move?
It's an unusual situation, no bright ideas at present but I would start by peering into the bell housing through the release arm hole and checking that the bearing is looking intact.
The end of the clutch lever typically only moves 3/8". Your having moved it 1" is a worry, particularly as you said that it went hard then easy. I fear you have "over centred" the pressure plate. This is not a good thing and can cause the pressure plate to fail. Can't think of any way to get it to pop back, assuming it's still in the moved position, short of pulling the engine & box.
|When disconnected from the release arm the slave piston will come out by itself, this is the spring inside the slave, which helps to take up any free play in the mechanical linkage that end to give a consistent biting point.|
When the slave does this it *should* be pulling fluid down from the master, so unless you top it up at that point then pushing the push rod back in to the cylinder should not cause the fluid to overflow. If it does then I suggest that as the push-rod, and hence the piston, came out it was actually pulling air in past the slave seal, because that can be easier than pulling fluid all the way down, especially if the master cap is fitted. Ordinarily it is fluid pressure that pushes the piston out of course, which presses the seal against the cylinder walls, but in this case the partial vacuum developed in the slave tends to pull the edges of the seal away from the cylinder walls, thus letting air.
No movement of the slave push-rod when the clutch pedal is operated and released indicated either air in the system or failure of the master main seal. The clutch is notoriously difficult to bleed, so air in the system would be my first thought. I've changed a couple of clutches and several hydraulic components and the only reliable way I have found is reverse bleeding i.e. link the right caliper and clutch slave nipples (they are the same size), open the clutch nipple, apply *gentle* pressure to the brake pedal, then open the caliper nipple and press the brake pedal very gently. Pressing it too hard will blow the tubing off the nipples. When the brake pedal reaches the floor close the caliper nipple before releasing the brake pedal, and repeat, Just a couple of iterations should be required, make sure there is space inside the clutch master, and keep an eye on the brake master so that doesn't get too low.
If the release arm is still easy to move, other than the free play, they I'd say you have buggered the pressure plate. Even if you can get it to pop back out I wouldn't like to rely on it having enough pressure to grip the friction plate against the flywheel adequately.
Thanks for the input. I removed the slave completely and found that when I pushed on the clutch pedal nothing comes out of the rubber hose. Put the slave back on. Not having a helper available I used the vacuum bleeder to pull fluid through the bleed screw. Will have to wait until Thursday when my LA is back from school trip to Europe and get her to pump the pedal. I will try the reverse bleed then.
Should I be able to push the clutch lever by hand. I can't budge it with out either a large adjustable wrench or a ratchet strap. Seems like a lot of force is required. I really hate the thought of pulling the engine to replace the clutch.
You can't push the clutch lever by hand, there is too much resistance. Remember that force is there to connect the engine to the wheels.
|I don't like vacuum bleeders, it's as likely to pull air in past the seals as it is fluid down from the master, especially the clutch slave which has a flat seal and not the type that fits over the piston as on the rear brakes.|
Your leg pushes the pedal about six inches, the slave pushes the release arm by about 1/2". To move the release arm directly you would need 12 times the force that your leg applies to the pedal.
Get all the air out of the system and the required 1/2" to 5/8" travel of the slave push-rod first, then you will know whether you have to dig into the clutch itself. Until you get the required travel you are wasting your time with anything else. However if the slave moves the release arm with the clutch pedal, but doesn't return with the clutch pedal, and successive pumps of the pedal push the piston out of the end of the slave (as it will), then you do need to dig in to the clutch.
When reverse bleeding by gently pushing down on the brake should the clutch pedal be fully down?
|After the whole system has been apart it is difficult to get all the air out without pressure bleeding - use about 20 - 30 psi. As the pipe goes uphill from the master cyl it traps air and pushing fluid with one pump is not enough to get it right down to the bleed nipple. If you can not pressure bleed then fill it up and then bleed with a few pumps to get fluid into the slave cyl then close bleed nipple, give about 20 quick pumps on the pedal & then leave for about an hour. then do it again - sometimes it needs another go. Quick pumps aerate the fluid in the system & the air in the fuid in the master lets itself out of the fluid whilst waiting. Have used this method many times - particularly on midgets.|
|J G Bagnall|
|"When reverse bleeding by gently pushing down on the brake should the clutch pedal be fully down?"|
No, that will block the bypass port in the master so you wouldn't be able to push anything backwards. It's why the clutch hydraulics develop pressure when the pedal *is* operated.
|My LA is back. Had her pumping the clutch and determined that the new master cylinder is not creating any pressure at all now. I must have damaged it some how. I had thought that might have happened when I could not get any resistance at all. I put a kit in the old MC and will try that.|
This thread was discussed between 07/07/2013 and 18/07/2013
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