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MG MGA - Master Cylinder Measurement

Does anyone have an original MC they can measure for me? I need the size of the two holes in the bottom of the reservoir. Thanks!
Steve S

Steve, they are very small! I have mine out of the car. Any suggestion on how to measure as precisely as you need? And can you not access your own? Just curious.
Art Pearse

I would use drill bits if your set goes that small.
Don Carlberg

There are 4 holes in the bottom of a master cylinder (2 for the brake and 2 for the clutch) - the very small holes are called the compensating orifices in the WS manual. I used fused wire to check they were clear when overhauling my master cylinder - if the adjusting rods are not set correctly these holes can be blocked off causing brakes to lock on.
Mike Ellsmore

Can't be 100% sure mine's original but I think it is and it has Lockheed on the side. The two holes are 3.10 and 4.75 mm diameter +/- 0.05mm.............................Mike
m.j. moore

Maybe you can scale it from this picture

Art Pearse

Thanks all, I should have noted that the brake side is the one I'm most interested in, although I think they are the same anyway.

Drill bits is the best suggestion I have for measuring!
Steve S

Using Art's piture as a guide. The larger holes feel like a 3/16" drill was used to make them. The smaller hole has 2 steps the big step is 1/8" but the mall hole in it's center is the hole that can cause problems. It is smaller than any drill here. It looks to be .020". Any out there with a # drill small enough to verify this? It looks like all 3 sizes are standard drill sizes.
R J Brown

Just curious Steve, why the need for the dimension?
Art Pearse

The tiny hole which is counterbored to 3.10mm is 0.68 +/- 0.03 mm..................Mike
m.j. moore

Art, I had a couple MCs sleeved and I think they were drilled to the wrong size. So, if I can figure out the factory size then I can correct them.
Steve S

Size of those holes is not critical, and probably has more to do with a reliable "smallest" drill in production situations than anything else. It is desirable that they be as small as possible consistent with letting free flow between the pressure side and the reservoir when brakes are OFF. The bigger the hole the more likely it is that the edge of the seal will catch and wear; remember that as that hole is closed off, pressure builds immediately, tending to expand the seal edge into the hole.
Position of the hole is more important. The seal must be clear at brake OFF, but close off the hole in the shortest travel possible. Both position and size determine initial travel before you get brakes. Some race suppliers supply different circlips or spacers to get the rest position of the piston/seal as close to that hole as possible, in order to provide maximum useful pedal travel and least waste motion.

FR Millmore

The hole size does matter, and affects bleeding which is part of my problem. Maybe not critical but a small change can make a real difference. I just want to make sure it's at least close!
Steve S

Steve -
Curious about your thinking here; what sort of measurement are you getting? And is the seal in fact clear of the hole at rest?

FR Millmore

Hi Steve,
I have just measured the holes in the bottom of my M/C reservoir, which I believe is original.The larger is approx 3/16" (4.7mm), the smaller 2-part hole approx 1/8" top half and 0.57mm bottom half.

Steve A.
S Ash

Well, the small hole is already larger than standard but I did find a couple other problems including variations in the size and hardness of the rubber cups. Even after fixing those issues, I'm still having an impossible time getting the brakes to function properly. The holes aren't blocked at all, hoses are new, and then master works perfectly on the bench. When plumbed in the system however, it just won't work and the piston returns as if there is a vacuum working against the return spring. I've swapped rubber, machined the pistons to ensure they aren't the problem, tried a pressure bleeder (which worked even worse than traditional bleeding) and everything else I could think of. I'm running out of ideas!
Steve S


I suffered much the same for a very long time and messed around with all the bits the same as you. I ended up binning the MC for a much better one made by AP Caparo using the Lockheed tooling. All my problems went away immediately. I often wonder if it was the bore poor finish by Lockheed due to lack of good quality control.

Dominic always says to replace brake parts if there is any doubt. I second that.

Steve Gyles

Trick is to combine the pressure bleeder with the traditional approach of someone pushing the pedal and you under the car at the nipples.

I just have to solve the mystery of why one rear brake is twice as efficient as the other. Have to do the safety check on Tuesday, and will fail on that unless I can sort it out.

dominic clancy

This thread was discussed between 20/04/2011 and 30/04/2011

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