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MG MGB Technical - Buggered my brake master cylinder

Yes, time for admissions of stupid mistakes again. I managed to ruin my just rebuilt brake master cylinder. This is the US style later tandem type with the pressure failure switch in it. I had a problem with a persistent leak from the end plug with the copper sealing washer. I'd replaced that and made sure everything was clean but I always had a small leak there. After a day or so I would always find a drop of brake fluid hanging under it. Someone at work told me those do need to be really tight and since I couldn't find a torque figure in the Haynes manual I thought I'll just make sure it was really tight. Don't you hate that feeling when you're torquing something up and you go just that little bit too far...

Anyway, the bolt now spins in both directions without going in or out so the thread is well and truely buggered. I then did what I should have done first and checked my better manuals. Bentley seems to have a figure of 200 ft inches. When I first read that I thought 200 ft lbs then realised that was silly. According to the ever useful Google calculator (type in something like "what is 200 inch lbs in ft lbs?" and it gives the answer) that is 16 2/3 ft lbs. My Autobooks manual gives a figure of 33 ft lbs.

What is the correct value? It's too late for my master cylinder now but I can get a replacement NOS unit I think. That's the problem with being an amateur mechanic. You sometimes have to learn from costly mistakes!
Simon Jansen

I'm sure we've all done similar Simon, I know I have ;-)

I'm not sure if these figures are relevant to your setup, but they're taken from the Haynes manual:

Master cylinder port adapters (later models) = 33 ft/lb or 45 nm ;

Pressure differential switch end plug (later models) 17 ft/lb or 23 nm.

HTH

Mike BGT

Hi Mike, I guess it is the pressure differential switch end plug? I shall have another look through my Haynes manual when I get home. I didn't see those figures the first time. 17 ft lbs does match what Bentley says.
Simon Jansen

Simon you've probably done this already, but just in case. I was told years ago that copper hardens over time. So always soften copper washers, even those fresh from the box by heating to red heat and allowing to cool slowly to room temperature just before fitting. Have done it ever since without thinking to confirm if it's good metallurgy or just an old mechanics tale.

Vic
V Todman

Vic. What you post is true and not "an old mechanic's tale".

Heat the copper washer to red hot in a torch. At that point, opinions differ. With steel, you would want to cool it in sand or ashes to allow it to cool as slowly as possible. With copper, I have always allowed it to air cool, as you suggest. However, others have cooled the copper washer under running water and report equally good results. They claim that the running water removes the oxidation that is a result of the heating. I have always, when using the water cooling method, had some remaining oxidation that I had to bead blast off. Hence, I use your method, let the part air cool, and bead blast it to remove oxidation.

Les
Les Bengtson

Simon,
Remember also to use a thread sealant ( Loctite or similar) when you replace the pressure switch as well as the copper washer otherwise you will still have a leak regardless of how tight you screw it in.

Annealling , by either method mentioned above , the copper washer helps to make sure it remains soft and acts as a "crush" washer.

Cheers , Pete.
Peter Thomas

Les, I am surprised at you bead blasting the copper washer after you have annealed it. I would have expected the bead blasting to have the effect of work hardening the surface of the copper leading to you loosing some of the benefit of annealing it. Please forgive me if I am wrong. I am not a metalgist, just a humble Chartered Accountant.

David
David Witham

Simon, before going to the cost of a new master cylinder, check with your local machine shop about installing a thread insert (helicoil in these parts).
John H

I ended up getting a new one. Well, new old one for a good price so this weekend I will fit that and re-bleed the brakes. The other thing I need to remember to do is test that the brake booster works. My old GT didn't have one and I haven't thought to try pressing the brake pedal when I have the engine running so I have no idea what it feels like.

Simon Jansen

OK, just an update for anyone who wonders...

I fitted the new master cylinder. I did open it up first on the advice of my parts guy. These are NOS cylinders so have been sitting for some time. The rubber seals looked OK to me but the two little shims were corroded. Sicne I had a seal kit anyway I replaced all the seals and also the shims. The ones in my new cylinder seemed to be brass. I had some good used ones from my original cylinder that seem to be stainless(?) so in perfect condition. Anyway, it is all installed and working with no leaks. I tried the brakes with the engine running and the booster does make a difference. I am also pleased that the brake pedal lines up perfectly for heel/toe braking (which I have been practicing in my every day Japanese car for months ready for the day I get to drive the B)!

Simon
Simon Jansen

This thread was discussed between 10/07/2006 and 16/07/2006

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