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MG MGB Technical - Nothing is easy...



I’m trying to rebuild my brake master cylinder in order to replace it with synthetic brake fluid. I’m hoping I have not scratched the primary piston too badly but now stumped with the secondary piston. My Haynes manual says “depress the secondary piston, withdraw the stop pin from the secondary feed port.” The diagram is not clear. Where is the secondary feed port? Is it one of the openings to the Reservoir or to the 4-way connector? How do you remove the stop pin? I’ve been probing all the orifices, is the stop pin supposed to just fall out? Should I be able to see it? How is it manipulated? Boy, am I confused.
Danny Jacob

Danny. It has been some time since I rebuilt a master cylinder. However, I seem to remember a pin, on top of the late model MC, which had to be withdrawn to allow the other parts to come out. You had to remove the reservoir to see this part. Perhaps others, who have done this job more recently, can contribute more. Les
Les Bengtson

Danny, I believe I remember that your car is a 70. If I'm correct the master brake cylinder for 68 to 74 1/2 cars does not have a stop pin. In your Haynes manual you should be following section 19 on page 155 rather than section 20.

That said, the tandem cylinder used in the 68 to 74 1/2 MGB is one of the most difficult master cylinders to repair. There is a spiralox ring under the return spring retainer, it's usually rusted and hard to remove without damage. As I recall the repair kit doesn't include a new spiralox. Next is the outer circlip item 16, it isn't too bad. After getting guide bearing item 21 and seal item 15 out there is a second circlip and it's very difficult to remove it without damaging the cylinder. You'll need a good snap ring retainer removal tool. To make things worse there is a step in the cylinder bore so it's almost impossible to hone the cylinder correctly. I did my cylinder several years ago and six months later it was leaking again.

I will not attempt to install a kit in mine if the cylinder fails, I'll pay the cost of a new or reconditioned cylinder. The MGB master cylinder is too difficult to remove and replace for me to take a chance on something that isn'd a pratical DIY repair.

FWIW, Clifton

Clifton Gordon

Thanks Cliff. Well I seem to be in mid stream now. Got most of that stuff out to clean, refurbish as I’m hoping to replace with synthetic brake fluid. May have damaged the spiralox ring I think you’re referencing, too. It was hard to get out. This project started because condition of the MC was unknown, it was drained of all fluid, rust was on shelf and down the side of engine compartment and the car was garaged for so long. I tried to flush the cylinder with denatured alcohol but signs of old fluid continue to drip out… that’s why I tried my overhaul. Will look again at the manual tonight… but may have already violated the “1st do no harm” philosophy despite working so carefully
danny jacob

Danny

While this is a lot of work, it is really the way to go. Once you are done and replace with the silicon, you should not have to work on the breaks again. Been nearly 20 years since I did the TD and the breaks are perfect 0 even with winter storage.

My 67 BGT will get this treatment this winter.
Bruce Cunha

This thread was discussed between 01/11/2004 and 02/11/2004

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