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MG MGA - Caliper rebuild

I have a pair of A calipers dismantled. I removed the pins and have some B pistons and a rebuild kit.
I have cleaned up the bores nicely, but I am having trouble ensuring the seal ring grooves are sufficiently clean and smooth. This is the critical sealing are , right? Any suggestions or should I get new calipers?
Art Pearse

The sliding seal surface in on the side of the piston, but the groove for the seal ring still has to be clean enough for the rubber ring to seat and seal. You might use a right angle screwdriver or a dental pick to clean the slot.
Barney Gaylord

Not getting any new calipers!
As Barney, I use an offset screwdriver, or I have made groove cleaners by bending a piece of heavy wire to a V shape, bending the ends at 90, sharpening same. Then squeeze it together, snap into the groove and twist around to clean.

FR Millmore

During a caliper rebuild what should be used to lube the seals and dust ring....I read that "Brake Assembly Lube" should be used but I have not seen such a grease..I have also heard of using brake fluid for this task ...any thoughts.
Thomas Koch


The seal ring grooves are not as critical as a smooth piston. Bead blasting or mechanical cleaning the grooves should do the trick. The inner groove holds the primary seal and the outer a dust seal. Since the inner seal is square it gets squished into the three sides of the groove when the piston is inserted. So unless the groove is damaged or has excessive pitting it is likely going to work fine. At worst it will leak and the caliper will have to be replaced.

If you are installing new brake pads and new rotors at the same time make sure that there is some free play between the pistons and the pads. I had to remove the anti-squeal pads that were glued to the backs of the brake pads to get enough clearance to insert the pads into the caliper. Even with that once I stepped on the brake pedal and then released it the brakes were dragging a bit the first couple of hundred miles until they bedded in and things loosened up. I suspect the small amount of pressure retained by the MC and tight clearances meant that the pistons could not retract quite enough until there was a bit more clearance. The dragging wasn

We used to use Girling red rubber grease for assembly outside the actual (wet part) cylinder. Don't know if you can still get it. Brake fluid is OK, BUT it does attract moisture, which causes rust under dust seals and such. Silicone fluid should not do that.
And there are now silicone assembly greases which should be OK for any fluid.

If the grooves are not absolutely clean, the seals will be too tight and maybe unevenly tight, which will causes dragging etc.

FR Millmore

I think that bead blasting in the grooves would be one of the worst possible things you could do... Not only will this not create a smooth surface for the packings to seat against, but you'll increase your chances of making the corrosion worse, depending on what lube/fluid you will be using. Remember that standard fluids attract water which will promote corrosion, and a bare, blasted surface will corrode quickly. That, and unless your masking/plugging are perfect, you are risking getting blast media into the fluid passages.

If you are determined to use a media blast the best would be something like walnut shells.
Del Rawlins

After placing the seals and ring is it proper to grease the piston up with somthing if so what? If I'm not mistaken it works best to install the piston last but I know you don't want any grease or lube getting into the brake fluid veins........any thoughts...Tom
Thomas Koch

That's what the Red Rubber grease is for. Outside of seals only - the fluid lubes the inside and seal face.

Just came to me - KY jelly might be good!

FR Millmore

Did moss stop selling their repro brake calipers? I bought one last year- that thing was worth $99 for sure.
Mark J Michalak

Moss had what they call new Calipers on sale last week for $93 each....but they are back ordered until I ordered MGB pistons and rebuild kits from SF which all cost less than $100. I'm about to put them back together to
night I'll let you know the results...........Tom
Thomas Koch

Mark, Moss have no stock of their brake calipers over here at least. I ordered one 2-3 months ago on the strength of your recommendation some time back. I've tried the other major suppliers over here with no luck. I had a catastrophic fault with one of my originals when the casting cracked on tightening up a bleed nipple (see pic).............Mike

m.j. moore

This seemed a deal:
N McGurk

Did not know about the repro calipers. or are they old stock?

Mike- that's horrible, some serious abuse at some point. Is there enough passage depth to drill it straight through, thread for an extension to fit the bleed screw? Probably 3/8-24, but that could go all the way into the bore if you use a copper gasket or Loctite under the adaptor.

FR Millmore

No, they're Chinese repros but other than not saying Lockheed they look very similar to the originals. I've put about 6k miles on one (my original's bleeder threads rusted away) that I had intended as a quick fix, but it works well and so far no issues, so I'll leave it on and worry about a Lockheed unit when I restore the car.
Mark J Michalak

FRM I don't know the history of the caliper but I guess a previous owner had had a rusted in bleed nipple, drilled it out oversize put in a plug and drilled and tapped with the original thread. Unfortunately the new hole must have been a bit off axis and the nipple didn't seal without excessive torque. The extra torque and lateral thrust plus the thinner body wall resulted in the damage. My son has taken it to an expert cast iron welder but even if apparently fixed I'll be uneasy about reusing it.................Mike
m.j. moore

Why bother repairing when new ones are relatively cheap - and will work reliably for years without any questioable seals or repairs

dominic clancy

Because, as I said above, recommended new ones are not available.
m.j. moore

I don't know how available MGA caliper cores are, but you could always get the MGB caliper adapter brackets. MGB calipers are cheap and easy to find.
Mark J Michalak

This thread was discussed between 05/08/2012 and 10/08/2012

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This thread is from the archive. The Live MG MGA BBS is active now.