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MG MGB Technical - Question about clutch slave cylinders

Hi,

I was wondering your views on either replacing the clutch slave cylinder or rebuilding with the moss kit.

I was also wondering about anyone's experiences with the cheaper aftermarket units. I have seen John Twist in his videos mention you cant bleed them correctly. I have also seen the Moss Motors video which says to disconnect the linkage and push the pushrod in to get rid of the rest of the air..

John Twist
http://www.youtube.com/watch?v=SRHUsCaK3LE

Moss Motors
http://www.youtube.com/watch?v=uj1dMdYgRK4
JRB Mr

The problem with rebuilding a slave cylinder is that you can't tell if it is rebuildable until you get it apart and check the condition of the bore. If it is only slightly pitted, you can hone it out and use the inexpensive rebuilding kit to get it back into operating condition. If the bore is severely pitted, it's time to replace the cylinder. The problem with the after market cylinders being difficult to bleed comes from them being shipped with the bleed valve mounted in the wrong hole to simplify installation in a smaller box. If you put the bleeder valve in the same place as the original, there is no longer a problem with bleeding the system. I always compress the slave cylinder piston, with a C clamp, when bleeding the system. It eliminates the pocket of air that gets trapped at at the top of the cylinder. RAY
rjm RAY

Hey Ray,

Great tip in regards to using the C clamp. I use one when changing brake pads... Did not put two and two together in my head. Sometimes when you have a culmination of perceived problems you forget you know what your doing...
JRB Mr

This pocket of air in the slave and not coming out of the bleed nipple keeps being mentioned but unless the car is in a major nose-down attitude it can't happen. The fact that the bleed port points downwards is irrelevant, it's no different to vertical runs of pipe and hose elsewhere in the clutch and brake systems. The drilling that the nipple screws into goes enters the cylinder at the very top of the angle between the cylinder wall and back face. When the car is nose up as would normally be the case when bleeding the clutch, or level when on a hoist, this is the highest point of the cylinder.

You have to be very careful when releasing the piston if you push it right back, the internal spring will push the piston out to take up all the play in the mechanical linkage between there and the clutch cover plate. This requires fluid to back-fill the cylinder, which has to be drawn all the way down from the master. It can be easier for the piston seal let air in than pull fluid down, negating the bleeding.

The problems with bleeding are usually associated with trying to push air down the long, vertical, relatively large bore pipe, especially with pedal bleeding, but continuous flow bleeding as with an EeziBleed often has problems as well. I've reverse bled several times, which only takes moments, which is pushing air out of the *lower* of the two ports in the slave, but I still get full travel. The easiest way of reverse bleeding is to interconnect the right calliper and clutch slave - the nipples are the same size, open the clutch nipple, with light pressure on the brake open the brake nipple, and allow fluid to flow slowly between the two. At the bottom of the strike close the calliper nipple before releasing the pedal, and repeat. Keep an eye on the brake master level as well as the clutch, and soon as you see fluid at the bottom of the clutch, usually accompanied by gurgling, you can close both nipples and top-off in the normal way.
PaulH Solihull

Hi Paul,

I ended up doing this... http://www.youtube.com/watch?v=kgpY_oGINU8

Works good. The clutch slave cylinder does not seem to be operating though... I havent put the c clamp on it yet as I cant find my big c clamp. I will give it a go tomorrow but if its bled correctly it should be working correct? Maybe there is some more air in the system... Im going by john twists video where he says that if the slave cylinder is not leaking then its not bad.
JRB Mr

"Works good. The clutch slave cylinder does not seem to be operating though... "

Hmmm!

"Im going by john twists video where he says that if the slave cylinder is not leaking then its not bad."

I changed a pal's clutch where the slave was working fine beforehand, but no matter what method I used afterwards (including pushing the piston all the way in where we *could* hear air going into the master, and letting it out very very slowly) I couldn't rid it of air, and believe me we tried as it was an hour plus trip to the spares place and back. A new slave bled straight away with the reverse method. Note that only the engine was removed, so the slave was not disturbed during the clutch change.
PaulH Solihull

Thanks Paul. Yea I tried with the c clamp to no avail. Will be buying a new one.
JRB Mr

Hi JRB,


I would recommend getting any parts you need from British Sports Car Restorations. They are a Moss distributor and they give up to 16% off Moss prices.

Their web site is www.bscr.com
KL Ken

I read somewhere that one of the problems with the slave cylinder is that the bleed nipple can be lower than the cylinder - the trick apparantly is to make sure that the bleed nipple comes higher - the principle being air rises upwards - or something like that anyway. Just a suggestion.
/Moss
Moss

"The drilling that the nipple screws into enters the cylinder at the very top of the angle between the cylinder wall and back face. When the car is nose up as would normally be the case when bleeding the clutch, or level when on a lift, this is the highest point of the cylinder."
PaulH Solihull

This thread was discussed between 06/12/2011 and 19/12/2011

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