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MG MGB Technical - brake master cylinder parts ??

after rebuilding the brake master cylinder kit, i installed the included caparo vacuum seal parts, vacuum seal, nylon sealing washer , and metal seal retainer. (they were not originally installed by previous owner when i started the rebuild !! perhaps i should have done the same. )

now,i can't adjust the mixture, too rich, and i can't adjust the timing, it is over 20deg btdc, it was 18btdc beforei started the rebuild.
so, should the vacuum seal stuff not be used at all ? or only parts of it ?

i am confused by the inclosed included instructions.
right now i am thinking of removing the vacuum seal stuff.
i really hope i have been clear



ps: from the instructions, i am not really sure which secondary cup to use. i used the new design because that was how it was configured when i got the car.

john sutter

John. I am not sure what you are writing of. "after rebuilding the brake master cylinder kit"??? By this do you mean that you rebuild the servo assisted master cylinder?

What are the "caparo vacuum seal parts, vacuum seal, nylon sealing washer , and metal seal retainer"? Are these the parts which go between the servo and master cylinder? I have never seen them as a part of a master cylinder rebuild kit, only the brake servo rebuild kit.

As to the timing, nothing you have done to the brake system could affect the ignition timing to make it non-adjustable. In fact, nothing that you mention should have had any effect on the basic dynamic timing at all. The basic ignition timing (taken with a strobe type timing light and with the vacuum line removed and plugged) is set at a certain engine rpm (1,500 rpm on a 79) and at a certain number of degrees before top dead center (10 degrees for a 79). If the vacuum line is not disconnected, and there is more vacuum present in the manifold/carb, it might be possible to see an increase in ignition timing. Similarly, if the engine rpms are increased beyond the stated 1,500 rpms we would expect to see an increase in ignition timing. But, nothing related to the work you describe should have any effect on the ignition timing, if taken properly.

The work you have done should have little, if any, effect on the mixture. The system was designed to operate with the brake servo unit connected to the intake manifold and to be fully tunable and capable of meeting the emissions standards in effect at that time. Simply rebuilding the brake master cylinder cannot have an effect on the air/fuel mixture.

So, a hypothesis. The vehicle had some form of air leak between the master cylinder and the brake servo which resulted in an excessively lean condition which was covered up by the use of an excessively rich needle in the Z-S carb. When this situation was corrected by installing the correct seals between the brake master cylinder and the servo unit, the problem went away, resulting in the fuel/air mixture now being excessively rich due to the mixture not being diluted by the air leak.

Air leaks, also, cause the idle speed to be lower than it would be when the air leak is not present. (That is why the idle speed increases when one sprays carb cleaner around the areas where leaks are present--it forms a temporary seal and enrichens the mixture.)

As I note, I do not fully understand what you are writing about. It would help if you could reference the parts you are writing of to the factory workshop manual or some other commonly available illustration. But, that being said, the symptoms you describe could result from fixing an air leak into the intake manifold and are similar to what is found when one replaces a bad intake/exhaust manifold gasket with a new one.

Les Bengtson

Confused here too. However as far as the servo goes if a fault, incorrect parts or incorrect fitting introduces a vacuum leak then that will affect mixture,which could have led to the DPO scenario Les describes. The servo should not allow a continuous flow of air into the inlet manifold, the only time any air flows is when operating and releasing the brake pedal, when a very small amount will pass through as the air valve opens and closes.

You need to explain just what you mean by "i can't adjust the timing", it's simply done by twisting the distributor one way or the other. 2 degrees different to before isn't very much, and on a car with manifold vacuum which this should be could easily be explained by a small change in idle speed, which itself could easily be explained by a change in mixture i.e. by sealing off a previous vacuum leak.
PaulH Solihull

hi, again

i use a gunson color tune, and no matter what i do the mixture is always too rich, really bright yellow.
the rebuild was of the brake master cylinder only, replacing the 4 seals( secondary cup, main cup, 2 of them, and reverse cup )thats all.

i have a timing light, what i am saying is that the the timing advanced from ~17btdc to well over 20 on installing the parts listed in the first post.

if you have ever used brake seals from moss motors you get the extra parts as listed in my first post. these parts go on the end of the master cylinder, the servo assembly is not touched.

so, can the installing of these parts account for my problem ?
hope this helps, sorry i was not clear in the first post, maybe i am still not

john sutter

John. My experience with the Colortune is limited--it never worked correctly for me with the fuel I have available. Even after the mixture had been set using a five gas analyzer, the color never came into the "correct, bunsen blue" range.

Nothing you describe having done, to your master cylinder alone with no other changes made, could have any effect on your ignition timing or mixture. If these have changed, you have introduced another variable that you, nor we, seem to be aware of.

Again, if you would reference the parts replaced to a common reference standard, such as "Factory figure M.1 on page 209 of the Bentley reprint of the factory workshop manual" it might be easier to understand what parts you have replaced.

Les Bengtson

Bear in mind John, that some rebuild kits have a plethora of parts! They are not all intended for use, just variations in certain circumstances or vehicles.Go back over your sequence of rebuild, see if you can copy the 'as before situation'. Theoretically you should have corrected this new problem. If not the new problem has nothing to do with the repair. Have you a 'book of words', the workshop manual looking at several referemces is sometimes useful. Good luck, Mike
J.M. Doust

This thread was discussed between 27/11/2011 and 28/11/2011

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