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MG MGB Technical - MGB Clutch fun!!

Still working around on this 74 1/2 B I have replaced the clutch master cylinder, filled the reservoir, crawled underneath, and attempted to bleed it through even to the point of removing the bleed screw, but no fluid is making it down to the slave cylinder.

What step of the process did I miss please? What steps should I take to rectify this?

Thanks

Paul
Paul Barrow

Two suggestions at first glance: You may need to "bench bleed" the master cylinder. I have had to do this with brake masters, but I don't remember clutch masters giving this kind of trouble. With the changes to equipment over the years you may need to. Two, have you blown out the clutch hydraulic line with compressed air and/or flushed with alcohol? Not knowing the running history of this car, it could be a buildup of crud in the line.

Paul Briggs
Paul Briggs

I just did mine and I reverse bled it. I hooked up a liquid hand pump (like the kind used to drain transmission fluids - you see then at chain auto parts stores). Filled it with brake fluid and pumped upward from the slave fitting - it worked for me. A slow and steady push on the pump handle got the fluid up to the top of the clutch master
r. LEARY

Flex lines deteriorate internally. The flex line maybe blocked.
Kimberly

Paul. As Kimberly notes, replacing the flex hose is a standard part of any system rebuild. When doing so, make sure you order the copper washer which fits between the clutch slave cylinder and onto the fitting of the line. The old copper washer, if it is in good condition, can be annealed (heated with a torch to red color, allowed to cool--either in air or under running water, cleaned, then reused), but, if it is not in good condition, it needs to be replaced. I keep a couple spares in my spare parts lockers so I will have then when needed.

The concept of reverse bleeding has worked well for me in the past. Pressure bleeding from the MC through to the slave cylinder has worked for others. "Pump and hold" style bleeding has also worked, but not as well as with other forms. Also requires an assistant in most cases.

Movement of the pushrod in the slave cylinder is a measured .365" average for five cars--about 3/8" when the system is working properly.

Les
Les Bengtson

Thanks gents for the input.

I dropped the slave cylinder and took it apart - boy was there a lot of black crud in there, no wonder nothing was moving!!

After a thorough clean off, and light crocus clothing of the cylinder and piston, and washing off the seal in brake fluid it works a reat ... just like new.

Once it was all back together normal bleeding was dead easy.

Now for a beer. Pull yourselves on each if you want.

Cheers

Paul
Paul Barrow

Paul Ba. Thanks for letting us know. Most people do not take the time to do so.

However, if you have not replaced the flex hose, and washer, it would be an excellent idea to do so in the near future. I have a good friend who is a very competent professional mechanic. He owns his own shop and spends at least one night a week in continuing training. As do all of the mechanics in his employ, for which his shop pays the training fees. In other words, a good businessman who is interested in turning out very good quality work and making sure that his employees are of the highest quality.

William and I have discussed how the brake fluid becomes dark. The basic fluid is a yellow-purple color. Rust, from the steel lines is a reddish color. Black comes from internal breakdown of the rubber hoses and the various seals.

As Kimberly notes, the rubber hose can crack internally and produce a sort of "flap valve" which only allows fluid to flow one way. Sometimes, it does not allow fluid to flow towards the slave cylinder. This might (there have been no experiments with this that I know of) account for some of the "master cylinder failures"--the power produced by the leg, leveraged though the clutch pedal, and applied to the master cylinder guarantees the internals are going to be moved, even if there is a blockage in the system. This may be a component of "master cylinder failure" which has not been discussed before.

If the flap blocks the slave cylinder from venting fluid when the clutch pedal is released, the clutch is not fully released. Sometimes, it is not released at all and the test for this is to crack the bleed nipple and see if the pushrod retracts when this happens.

It is easy to visualize a semi-blocked condition which puts excessive strain on the slave cylinder (never fully releases) and the master cylinder (never is capable of developing full power) due to such a partial blockage.

Until your thread, I had never, fully, thought about this. Your problem, and Kimberly's response, started a train of thought. Perhaps, a useful one.

It has been recommended that the brake fluid in the clutch and brake systems be flushed and renewed every 24 months.

The flex hoses, as their name implies, flex in operation. This leads to a more rapid breakdown than one would expect with the internal seals of the various master and slave cylinders. The flex lines are unrestrained in their flexing while the seals in the master cylinders, clutch slave cylinder, rear wheel cylinder, and the calipers are restrained in their ability to expand.

It would be interesting to note how long a new master cylinder, and its associated "slave cylinders" (both within the clutch and brake systems) would hold up if we were to replace the hydraulic fluid every two years and, at the same time, replace the flex hoses in the system.

Mayhap, worth considering. It would be worth hearing from anyone in a position to set up such a test. I have two 79 mgb roadsters, one of which has a recently rebuilt brake system and one of which would benefit from a brake rebuild. Both have had new master cylinders installed within the last year.

Thus, I will begin the experiment. The help from others who are able to do a similar experiment would be appreciated.

Thanks again, Les
Les Bengtson

This thread was discussed between 20/07/2007 and 21/07/2007

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