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MG MGB Technical - clutch bleeding

'ello all,
i have a 79b, daily driver. i rebuilt the clutch master cylinder this winter, but not the slave. now i'm trying to bleed it with no success. i have done it by the shop manual, ez-bleeder, paul hunt's reverse bleed via brake, all to no avail. (now the brake warning light is on.) while attempting to engage the clutch with the engine off it seems to work, but not with the engine running. and bonus, at one point the master cylinder cap top came flying off spraying fluid across the fender!
i've searched the archives and tried what i've found. two winters ago i rebuilt the stromberg carb and she idles smoothly at 900 rpms........thought i'd manage this too.
any advice?

Brady, In my experience the best way to bleed the slave cylinder is to unbolt it from the trans, compress its piston until it bottoms out with a "c" clamp and then bleed the slave. This removes the residual air that normally gets trapped inside the slave cylinder that causes a soft pedal. RAY

Brady pump the hydraulic fluid from the bottom. I generally connect a single pint plastic container with a rubber tube and just squeeze - never fails.

Bleeding from the slave upward worked for me too. I filled a hand suction pump with fluid and connected it to the slave bleed fitting. Worked the first time.



The brake warning light is on? That should be totally isolated from the clutch hydraulics.

How do you know the clutch is working with the engine off? Either you can feel back-pressure or you can't, which *is* obvious, but if you can feel back-pressure the only way you know the hydraulics and clutch are working properly is if you can engage reverse with the engine running without grinding.

If pumping the clutch pedal blew the cap off, then it sounds like the master valve is pressurising the fluid reservoir and not the lines, i.e. a major error in the rebuild.
Paul Hunt2

Although the factory service manual describes a procedure for refilling and bleeding the clutch slave cylinder after it has been installed onto the bellhousing, it is easiest to bleed it before attaching it to the bellhousing. The clutch hydraulic system is a real pig to bleed because of the long vertical section of pipe with the U-bend at the top. The conventional bleeding technique requires that any air in the pipe must be pushed all the way down the relatively large-bore pipe before it can exit the nipple of the clutch slave cylinder. This is difficult enough with a continuous pressure bleeder connected to the clutch master cylinder, and well-nigh-on impossible when using the old fashioned technique of using the clutch pedal to pump the hydraulic fluid through the system. Failure to get all of the air out of the clutch hydraulic system will result in the clutch having a low biting point, or even failure to fully disengage. Unless you have either rebuilt the hydraulic system or previously flushed out all the old crud and old fluid with denatured alcohol, do not reverse bleed the system without flushing it out first, otherwise you will push the crud into the clutch master cylinder. Use a C clamp in order to prevent the piston of the slave cylinder from popping out, and then use either a gunson’s EZ Bleed or a Mighty Vac tool to refill the system through the bleeder nipple on the clutch slave cylinder. This method works best because air bubbles tend to rise upwards. After bleeding the system, reach up and push the actuating rod all the way back into the slave cylinder, and then bleed it again. This pushes any air left in the cylinder back into the line that goes up to the clutch master cylinder. Rebleeding then expels this air. If you do not have either of these tools, then a more primitive, but by far easier way to bleed the clutch hydraulic system is to cross-connect the nipple of the right front disc brake caliper and clutch slave cylinder bleed nipples (they should be the same size), open both, and use the brake pedal gently to bleed, or even fill, the clutch. This is even easier than using a Gunson’s EZ Bleed, and with either technique you should get the full travel of 1/2" to 5/8" of the clutch slave cylinder push-rod. When rebleeding the system make sure that you siphon some fluid out of the clutch master cylinder first, and make sure that you keep an eye on the brake master cylinder and not let it get too low.

Another cause of low biting point is wear in the master linkages of the push-rod, the pedal and the clevis pin. Note that wear in the mechanical linkages at the slave end is not an issue as the design of the hydraulic system compensates for such wear. If the biting point is very low, then this will also make gears difficult to engage as the clutch is not being fully disengaged. In this case you will get grinding when selecting reverse. No grinding when selecting reverse usually means problems with the gearbox or selector mechanisms.

A “fierce” clutch means that it is snatching with very little slip. This is different from a low biting point, and can be caused by oil contamination of the friction plate that is produced by a leaking rear crankshaft seal. This is usually revealed as oil dripping out of the hole at the bottom of the bell-housing, which should contain a cotter pin to keep it unblocked. Note that a leaking front seal on the gearbox will also result in a leak from this location, but is less likely to contaminate the friction plate.

If you feel vibration when the pedal is fully depressed this can mean the release bearing is rubbing on the cover plate, and will eventually wear through and break. Vibration as you operate the pedal could be rough surfaces on the linkages at the clutch master cylinder or clutch slave cylinder. Vibration when the pedal is partially pressed and held in a fixed location could be problems with the flywheel, cover-plate and friction plate surfaces, or the pilot bushing (in the end of the crankshaft), or the input shaft of the gearbox that fits into the pilot bushing. Judder as it takes up the drive is something else again, but can also be caused by these components.
Steve S.


If I understood correctly, you said your clutch pedal feels normal when the engine is not running. How long did your car sit? Maybe the friction disc stuck to the flywheel?

If so, you will need to fasten or hold the clutch pedal down to the floor, start it in gear, and drive around like a madman giving it gas and brake at the same time. Do this in a somewhat open area so you don't smash into something.

C R Huff

Because of the long vertical section of relatively large bore pipe and the inverted U-bend at the top it *is* a pain bleeding the clutch as has been said. A continuous pressure system like the EeziBleed should get round this but it doesn't seem to. However it is far easier and cleaner to reverse fill or bleed the system after all the components have been installed than it is to pre-bleed either master, slave or the pipe. I've done this in two ways - one was with the EeziBleed on very low pressure connected to the slave nipple, the other was by linking the right-hand caliper and clutch slave nipples (they are the same size) and using the brake pedal to reverse fill or bleed the clutch. Both got the correct slave push-rod travel in seconds, the latter way being easier and quicker. If bleeding the clutch rather than filling it you do have to remove some fluid from it first to prevent overflowing, and if you have the single-circuit tin-can brake master then you have to be more careful to check you aren't emptying that.
Paul Hunt2

I fit my mityvac to the master with a fitting, and pushed it down. Then I did a couple pumps of the clutch and bled it the old fashioned way, and it's been fine for months.
don g

This thread was discussed between 10/05/2008 and 14/05/2008

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