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MG MGB Technical - Removing Master Cylinder

A repair manual I have recommends (for a '72B) to removing both the cluch and brake master cylinders as a unit with the pedal box. Is this true? I was planning to have only the brake master cylinder rebuilt (it drips down onto the brake pedal), but if I have to remove both masters why not have both rebuilt.
Joe

Joe. I have removed the brake master cylinder without removing the clutch MC several times. Therefore, if it is your desire to only rebuild the brake MC, it is fully possible to do so. If, however, you have any doubts about condition of the clutch MC, rebuilding it is rather easy (especially compared to the brake MC) and it would not hurt to do so. The clutch MC requires that you remove the rubber plug under the dash, stand on your head and try to remove the fitting on the forward "snout" of the clutch MC. To me, this has always been a difficult and time consuming task. before you do either task, make sure you have all of the necessary parts. Examine the Moss catalog, pages 93 and 112 in the newest catalog, and look for the copper washers required for proper sealing of the "banjo fittings" to the bodies of the master cylinders. I have never received these with any rebuild kits I have bought. These either need to be replaced with new copper sealing washers or, if the old washers are in excellent condition, they may be annealed and re-used. (String on a piece of wire, heat to bright red in a propane torch and cool. I like to let them air cool, then bead blast them. This leaves them "dead soft" and they will seal quite well. Others quench them in water, which removes most of the oxidation, and report no sealing problems when re-using them.) The copper washers are the part nobody remembers until the system is leaking and they ask, "Did you remember to replace the copper washers?". Les
Les Bengtson

Just a quick helpful hint: at Home Depot you can get a phillips screwdriver with an 8 inch long blade that will reach right down in between the pedal box and fender. It's a heck of a lot easier than trying to get big hands down in there or dropping the screwdriver down inside the fender (maybe this should be on the 'done anything supid thread?') ;o)
Ken T
Ken Thompson

Ken, the only problem with the long screwdriver is that the screws, if original, aren't Phillips heads, they're Pozi-Drives. If they are even a little bit corroded the heads will strip with a Phillips head driver. I use a 1/4" drive ratchet, a long extension and a Pozi-Drive bit taped into a 1/4" socket. Haven't ruined one yet!
Ken Lessig

You probably mean that the slots strip out, and that explains why so many people have trouble getting those screws loose. Last year I took everything out of the engine compartment for a repaint, and replaced as many of the screws as possible. Are there markings that distinguish the pozi-drives from regular phillips? Are the replacement screws from VB or Moss actually pozi-drives?
Joe, Back to the thread of the master and clutch cylinder, if the cylinders have any inside egg-shaped wear or gouging, you might be better off to buy new cylinders. More expensive but more reliable. I bought a rebuild kit for the clutch master, honed it, and it still leaked internally and still had to buy a new cylinder. Have your local MG expert take a good look down inside each cylinder first.
Ken T
Ken Thompson

Joe,

I agree with Ken T. I tried to rebuild both my brake master cylinder and clutch master cylinder on my 74 and they both leaked after rebuild. I honed the bores and they looked smooth and pit free, but did not work well. Regardless of what I did or how many times I bled the brakes they did not function as designed. New ones work so well you don't need any servo assistance for great brakes. If I was you I'd replace both with new units and be done once and for all.


Best regards Andy
Andy Preston

Just a comment relavant to Ken T's and Andy's comments: I have encountered very few things in this life which are infinitely rebuildable, and brake cylinders, either wheel or master, are among those things better off replaced. Just my experience.

Honing, etc can take off enough material to cause leakage from brand new seals even on the first time around for rebuilding. And if you do not know the history of your cylinders, YOUR attempt may not be the first, and therefore, you may already have passed the point where new seals will find a bore small enough to seal properly.

That is why there is a business in resleeving obsolete cylinders which need to return to the OE size bore to make them effective again.

As long as there are brand new, off the shelf units which are far newer even than NOS and made to the OE specs, I will continue to replace mine, even tho I know that costs me more $ and I may miss that golden opportunity to rebuild yet another hydrualic cylinder. The certainty of safety and the waste of new brake linings by leakage has to be weighed against the money saved by rebuilding, I think. JMHO.
Bob Muenchausen

After some research, I have decided to the MC resleeved with brass and bored. I have chosen Sierra Specialties in CA who had good references and seemed knowledgeable. It will save me about $50, and is guaranteed not to leak. I also decided to purge out the silicone fluid the previous owner installed and go to DOT 4. I haven't had any serious problems I can attribute to it, I assume the wheel cylinder leaks and MC leaks are because the car is 30 years old.
Joe

This thread was discussed between 13/10/2002 and 17/10/2002

MG MGB Technical index

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