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MG MGB Technical - Bleeding servo brakes & broken press warn switch

I just replaced the front calipers on my '67 CB V8 conversion that has a post '75 ('76?) servo and MC.

I've twice tried to bleed with an EZBleed but end up with spongy pedal and pull to left.

I was told to try again but remove the white plastic warning switch (which is not wired up) first.

Of course, the switch broke off level with the MC surface. The part that came off had the center plunger intact but the threaded portion and whatever else is inside the MC will not come out by any means, including a screw extractor.

I plugged the plunger hole in the plastic (which is now somewhat enlarged)with a machine screw that seemed to bite into the plastic ok and make its own threads.


Will leaving the broken switch part in place cause any safety problems? Is there likelihood of fluid leak from the bodgy plug job? Will leaving it in place make it difficult or impossible to successfully bleed?

Any suggestions as to how to sucessfully bleed? Do while running engine and use the traditional 2 person routine? Something else?

Thanks and sorry for the long post.

David Hawkins

David. There is a second chamber in your master cylinder--the one which the brake balance switch operates off. Look at the illustration in your workshop manual. If the rubber O rings in the balance section go bad, you can get air into the main chamber of the master cylinder and it will never bleed properly.

The hole for the brake warning switch was, I believe, a 5/6" thread. I would have to check to be sure. It might be possible to clean the hole up with the proper tap. I have sealed the hole with a machine bolt and some sealing compound when I had the warning switch side go bad and it worked until I was able to get a new master cylinder.

When I bleed the system, I start at the left rear (longest run of brake lines), the right rear, the right front and the left front. I find that having an assistant makes the job much easier for me, but know others who vacuum bleed and claim good success.

Spongy pedal indicates air in the system from some source. Pulling to the left indicates the right wheel is not being braked as much as the left wheel. You can check for proper functioning of the disc brake calipers while you are team bleeding the system.

Were it I, I would consider replacing the master cylinder. I have done this on two of my cars with the thought that a new cylinder will have several rebuilds in it while my original cylinders might not. With a new MC, I can keep track of when the brake system is flushed, rebuilt, etc. over the next many years. With an old MC, I do not know where it has been nor how it has been maintained. If you have gone to the trouble and expense of a V-8 conversion, skimping on something as important as brakes might be foolish.

Did you rebuild the calipers? Do you know when they were last rebuilt? Could the pistons, pushed back into the calipers when installing the new pads, have carried some grit or rust into the system? Worth considering and checking. Les
Les Bengtson

Les: Thanks for you reply.

Both calipers are "new" (rebuilds from Britek).

Last season I had 2 successive right front rotors warp and though the brakes were working perfectly, I was hearing ticking from the right front that was getting worse. Thought that I might have a sticking right front caliper so replaced both over the winter (and rotors again). However, in removing the right front wheel, it became apparent that the outer wheel bearing was shot. So, I replaced the front wheel bearings.

So I am going on the (perhaps erroneous) premise that since the MC (which was "new" in 1997 when the car was restored/converted) appeared to have been working fine at layup last fall, it probably is okay now. Except for breaking off the warning switch of course.

Do you think that leaving the broken switch part in place but plugged will cause any safety problems? Will leaving it in place make it difficult or impossible to successfully bleed? i.e. lock the pressure differential piston, or whatever its properly called? Is there likelihood of fluid leak from the bodgy plug job?

Did you mean a 5/16ths thread? Coarse?

David Hawkins

David. What brake fluid are you running? The standard fluid most of us use is Castrol LMA which will absorb moisture. Hence, the practice of draining the fluid every two years. (Some also believe in flushing the system with alcohol which will help remove any trapped moisture, but not all have recommended this. In a high humidity area, it might be an excellent idea.) So, if you have not changed your brake fluid regularly, you may have some corrosion in the MC which could have damaged your seals. After 8-9 years, a master cylinder may be in need of rebuilding. Hard to say for certain without removing it and doing a tear down inspection.

As to the plugged switch opening, I cannot tell you if it would be safe or not. My only experience with this was on my daughter's car. After we purchased it, I found brake fluid leaking from the switch. It should not have been as there are two rubber O rings which should have contained the fluid. I also noticed the car had been in for "brake problems" four years running. (The previous owners keep all of their repair records from The MG Shoppe in Tempe.) When I took it apart, the main bore had been rebuilt, but the bore for the rod which moves the brake warning switch had not. You may remeber the "Wiz" posting about his problems with removing this piece in a previous thread. My solution was a short machine bolt and some sealer to make an air and fluid tight seal until I could get a new MC. Personally, I would not drive a car myself that only had a bolt threaded into the remains of the plastic switch.

If you decide to pull the MC, e-mail me, or post, and I will dig out a spare master cylinder and determine the exact thread size. I remember it is being a 5/16" UNF bolt, but could be wrong. Since you would want to run a tap into the hole after getting the old switch parts out, you would need to have the exact size. But, if you are not going to do this, it would be a waste of time to dig through my parts shed to find the old MC. Let me know. Les
Les Bengtson

The bolt IS a 5/16" UNF. I used a copper washer on the bolt to seal potential leaks.
Jeff Schlemmer

Les and Jeff:

Thanks for the replies.

Les: I have always used Castrol LMA. Am waiting for a new batch before my next bleeding attempt. (By now I've probably bled enough fluid through to qualify as a "change"). There seems to be no leak at present from the switch hole so I think that I will leave the MC in place for now but thanks for the offer to dig.

Bob at Britek strongly suggested that I not plug the switch hole to be air and fluid tight as the original switch fitting has a small vent incorporated into it. I have tapped the plastic piece still in the MC for a #10-32 hex bolt and hacksawed a longitudinal "vent" down the bolt and will try this as a plug. If I detect any leaks then, as a temporary measure until MC removal and rebuild, I will use an "unvented" bolt sealed as Jeff has done.

David Hawkins

This thread was discussed between 27/03/2005 and 29/03/2005

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