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MG Midget and Sprite Technical - Caliper questions?

Yesterday I ran out of gas, and while coasting as far as I could, I heard a grating noise from the right front.
Today I took apart the disk and bearings to inspect.
I suspected a bad caliper because they are among the few original parts. The pads seem to be worn evenly, and I have braided brake lines so I don't suspect line swelling.

My main concerns are that the calipers don't retract when I let off the pedal, and the outboard piston seems to move a lot further out than the inboard one.
Is this normal?

I'm looking at Centric brand re-manufactured calipers, any thoughts?

P Burke

Pistons on disc brakes don't retract like the shoes on drum brakes. There are no return springs. The pads just ride along the disc. As for the distance that they protrude, it depends on the centering of the disc in between the pads. If the disc is centered a bit to one side or the other that will cause the pistons to compensate one way or the other. If you pushed both pistons all the way back in and then stepped on the peddle they would travel the same distance barring any internal problems.

From the looks of your picture, I would consider replacing the pads and possibly the disc too depending on how thin it is.
Martin Washington


That isn't true, the main seal is lozenged when brake pressure is applied and grips the piston so acts to pull the piston back slightly when the pressure is let off.
David Billington

I stand corrected. But my guess is that the movement is almost unnoticeable. Not like shoes.
Martin Washington


The movement is small but enough that no pressure should remain forcing the pads onto the discs. Well that is how it should be, I have experienced partial brake binding on the odd occasion when the pads and disc were all worn low so maybe not optimal operating conditions, a brake service and things worked well again.
David Billington

That's what I wanted to hear.
I've not noticed the noised before and the pads are worn more than 1/2 way.
There was no (roller) bearing damage and no other evidence of anything else rubbing.

I'll put it all back together with new pads and bearing grease and report soon.

P Burke

Possible of course that you had picked up a bit of grit that accentuated the scraping noise.
Guy W

While you have it apart, I'd check the thickness of the rotors just to make sure that they're not too thin. No need to take everything apart twice.
Martin Washington

Lawrence Slater

I've recently had EXACTLY the same issue, both on my midget, and my Sprite.

Both pistons free to move out, but not easy at all to push one of them back in.

Solution? New pistons and seals. Very easy and cheap job to do. No need to split the caliper. 16 quid per caliper (collected because he's not far from me). But no doubt you can get the pistons and seals in the US too.

Of course it comes with the caveat that, --

I'm an uneducated, self taught, untrained twat, with no brake engineering qualifications at all, who thus doesn't have a clue what he's doing, and so shouldn't be offering ANY advice to anyone, esp' about safety critical issues like brakes, -- when there are professional people out there, who know far more than me, and you won't have to be very lucky to get away with it. -- LOL.

Oh. And feel free to vote for whoever you like and believe in any fantasy you like too. I wouldn't want anyone thinking I'm telling you what to do about anything. -- As if my word is instantly obeyed, and nobody here thinks for themselves. -- ;).

Lawrence Slater

Looking at the state of the end of that hose, I'd be changing all the flex hoses while you are in there.
dominic clancy

Actually the main culpret is probably rust

the midget piston is made of mild steel and not stainless steel so a little moisture around the seal ring and let to sit for a few week is going to have an effect on the pistons ability to retract

they do make a replacement stainless steel piston

i had this happen to me... and has happened to others in the archives and being your in Florida, home of both exotic reptiles and high humidity id think rust is not considard uncommon

Prop and the Blackhole Midget

But even plain steel lasts a long time Prop. Mine were the original pistons, and lasted almost 50 years.

How much for seals and Pistons in the US?

Your right about where the rust and crap collects in the caliper. As you say, between the outer and inner seals. And then when you pull extract the pistons you see all the rust on the pistons you don't see from the outside.

On my Capri. I put in new pads. Pushing the pistons back in, caused the seals to leak, because now the seals were trying to seal against previously exposed and thus rusted areas of the pistons.

Surprisingly the seals themselves, on both my Midget and Sprite, and on my Capri looked completely ok. I could have just replaced the pistons, but since the seals are so cheap and it's easy to do, I changed the seals too.

One thing though. The outer seal on the Spridget caliper is retained by a steel ring that is pressed in with the outer seal. 1). I found it FAR easier to fit the new steel ring retainer, BEFORE inserting the new outer seal. 2). Next tme I have to do it, I won't automatically change the outer seal steel retainer, because it's fiddly, and the retainers were ok and didn't really need changing. Just extract the outer seal, clean the exposed groove, and fit the new seal.
Lawrence Slater

Some pistons are chrome plated. But then if the chrome does get scratched rust gets in and spreads, lifting the plating and destroying the seal. I also somewhere have a pair of plastic ones! That heavy duty "engineering" ABS plastic used for industrial applications. I never used them as I couldn't see them dissipating heat from the pads effectively.

I have got lazy in recent years but as part of a 2000 mile service I always used to clean the exposed band of the calliper pistons by spraying with WD40 and dragging a pipe cleaner around between the back of the pad and the piston seal. It was quickly done whilst I had the front up to grease the king pins.
Guy W

" I always used to clean the exposed band of the calliper pistons -- "

Very good idea. I reckon they'd last double time if that was done regularly.
Lawrence Slater

I would use a brake overhaul kit and simply polish up the pistons with emery cloth - unless they are well gone.

Unlikely to be the caliper.

Pads....If still well within limits, leave alone.

Should be cheap enough and easy to do...RTFM. Personally I use silicone grease on the outside of the pistons before reassembly
Mark O

Is silicone grease compatible with dot 4 fluid? I used Girling red rubber grease.

I'd have polished mine too, but the corrosion left the pistons pitted.
Lawrence Slater


Silicone grease is pretty inert and I have used it on all manner of brake pistons with no issues, but fair to say I do not know for certain. It is however excellent stuff for repelling water so I will take the risk.

Pits on the surface would constitute replacement in my book also, but in my experience, surface rust can be removed and the piston replaced quite satisfactorily.
Mark O

I've got some silicone grease too. Definitely good for repelling water as you say, and doesn't get washed away easily.
Lawrence Slater

I changed mine for Stainless Steel pistons from Minispares. Piston issues solved forever
Rob Armstrong

This thread was discussed between 14/02/2016 and 16/02/2016

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