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MG Midget and Sprite Technical - Odd binding brake

Hi all,

Replaced my calipers and pads last year but have barely driven the car! (Bought house..) but now I have it back out I have 2 issues, on is a hot misfire, the other the driver's side brake is binding.

The brake is binding to the point it won't move! Obviously gets really hot and everything expands. The thing that makes it such an odd one is that it will suddenly release.. when it first did this I thought it was because I had let it cool and something had settled down. The next time I got annoyed and pressed really hard on the brake and heard a knock and felt the pedal free up (pedal pumps up to being rock hard) this time all I did was reverse off the road about 2meters..

I have has a quick look over with the wheel off and installation looks fine, I have tried bleeding the brakes again, nothing. It has braided brake hoses so I can't believe it is the inner creating a 1 way valve like some stock hoses do..

Anyone had anything like this before? Maybe I have fitted something slightly incorrect (did same thing on passenger side and had no issues)

Karl Bielby

probably muck or corrosion on a piston due to the long lay-up. Time to do a rebuild on that caliper I'd say.
David Smith

On brand new caliper? Or a year old but probably 100 miles max use.. garaged the whole time.
Karl Bielby

Yep Karl I got the same on my nearside front. All clean and new. Brake cleaner didn't work after several attempts. Finally eased the seal out and skooshed it with PTFE spray and worked the piston in and out carefully.
It stuck one more time so repeated and seems okay now.
Hope that helps.

when it sticks on try cracking open the bleed nipple on that caliper. If this releases the siezed piston then the fault is hydraulic. If not, then it is the piston itself sticking as David and Rib say. My money is with that too.

No cost, not much effort, very extreme long shot - you could check the vent hole is clear on the master cylinder cap (no doubt I was the only person in history to get a cap without a vent hole).
Nigel Atkins

When I had brakes sticking on at the front I checked the discs and found them too thin so put new ones on and it sorted the problem. Whether it was the discs or whether pushing the pads and pistons back in I can't say but good to check all is in spec. I suspect the wear was causing the piston to skew slightly and stick. The discs aren't very thick when new so worth a check.
David Billington

Cheers for the replies fellas, that thought had forward my mind Nigel! Taking the cap off doesn't have any affect it seems!

I had the bleed nipple open with a clear hose on when I was bleeding and could see the fluid moving nicely when I put pressure on the disc either way, so I think the hydraulic side is good.

I will check that out David, I think they are all good as I remember not changing them at the time of the overhaul as they were good.

Reckon it looks like I need to get the caliper of again then! Pop the seal and give it a good spray up, or maybe a few drops of brake fluid? And a good bit of excersising.

As well as trying to sort the misfire!
Karl Bielby

Well I changed a few bits to try and sort the misfire, so wanted to take it for a spin. Before I did I sat there stopping on the brakes. Then jacked it to see if the wheel would spin, which it did. Brake drags ever so slightly, and disc does appear to have a slight warp.

Took it for a spin and had no issues with the brake at all... Wonder if it has freed off now that I have used it slightly more often? I did also loosen the reservoir cap slightly, just incase. But I did check the hole was clear! (Actually had fluid in-between the two layers..?)

The miss has improved, but not fully disappeared, might have to start a thread now as I'm stuck!
Karl Bielby

Took the driver's side caliper off and pads out. Pumped the pedal to push the pistons out and now they are well and truly stuck out!

Not sure what to do now.. check if they are in warranty date with Moss or just pull them in half and rebuild...

This will also explain why the pedal wasn't returning as it used to.
Karl Bielby

see Grey's post on 18 June 2018 at 18:45:02 and wind or gently push the pistons back in before/during/after as possible.
Nigel Atkins

Very good shout! Totally forgot he had mentioned that!
Karl Bielby

Did a brief bit of faffing the other day, I suspect the hard pipe causing restriction or a one way valve type of affair.. maybe the flare is damaged or something.

Even with the brake removed, there is a fair amount of resistance in the pedal, and it won't return (pedal stays down)

Now to find some pipes, or make some up!
Karl Bielby

Well it definitely is odd...

Had callipers off, pumped the pistons out a fair way and excersised them a good few times and all seems good.

But I have found that the brake pedal itself is very stiff on the shaft. The clutch pedal is much looser. I also noticed that then I move the brake, the shaft bolt head moves about, as though maybe a bush has worn or come out. So the upshop of all that is that the pedal isn't returning, thus leaving the brake applied slightly.

Will have to check out some exploded drawings to see what's going on in there and how to disassemble.
Karl Bielby

This is a fairly common problem. The brake and clutch pedal both pivot on a single bolt going through bushes in each one. The bushes dry out and seize up. The long bolt screws into a captive but and needs to be undone and removed, allowing the pedals to be removed from the pedal box once the clevis pin to the master cylinder pushrod has also been taken out. Then clean up, regrease and reassemble. The catch to all of this is that the long pivot bolt doesn't quite clear the bodywork with the pedal box in situ so this needs to be undone from the footwell top and raised up so the bolt can be removed. A real pain in the backside!

When I did mine I made 2 modifications. I ground down one of the hex head flats on the pivot bolt and it can now be inserted and removed without releasing the pedal box. The other was to fit grease nipples to the pedal pivot bushes.

What a pain! I did think that when I was looking at it, there is some kinda flange in the way isn't there.

Do I need to disconnect the hydraulics to get enough clearance? God I hope not! I'm sick of bleeding brakes now!

Also, is it just a case of lube it all or do things usually need replacing?

Karl Bielby

You might be lucky and be able to douse the pedal pivot bush repeatedly with penetrating oil and work the pedal up and down and it might free up. If the pivot bolt is turning with the pedal try holding the bolt head firmly with a well fitting spanner as you do this. You may even need to wedge the spanner in position and use your foot on the pedal to get maximum force onto it. If you can get the pedal to move without the bolt turning with it you are 90% of the way there!

If you do need to unbolt the pedal box then with care you can raise it without disconnecting the hydraulics but be gentle with it! Even then the problem will be getting enough purchase to undo the pivot bolt with the pedal box waving around unsupported. You are really better off geting it to free up whilst the pedal box is firmly supported in position

When I was excersising it earlier I'm pretty sure it only moved a little and some movement of the pedal around the bolts was happening. It is just really quite stiff. Will get me head down there again on Friday.

Worst case, I guess you could take the nut off the bolt before removing the pedal box.. might make it easier, then drift the bolt out.
Karl Bielby

Another thought, is is possible to remove the master cylinders from the pedal box without disconnecting hydraulics? So remove the pedal box but leaving the cylinders still on the car..
Will get a loser look Friday.
Karl Bielby

The nut is welded to the pedal box frame; it is the bolt that undoes.
And no, I don't think you could leave the m/c's in place and remove the pedal box. The pedal box has flanges folded under the m/c which would prevent you lifting the frame clear.

Ah rubbish... Hard way it is then!
Karl Bielby

All sorted! Cheers everyone.
Karl Bielby

So, c'mon, what was it then?
Nigel Atkins

Pedal sized to the shaft!

Lifted the box and got to the bolt and freed it off. Not 100% perfect but it's working perfectly now!

Can't believe all of the faffing I went to with the calipers off and on several times, when it was just that! I had read about it previously too, but when I looked, the bolt wasn't moving in the slightest!

Ah well.. MOT time now!
Karl Bielby

This thread was discussed between 18/06/2018 and 03/08/2018

MG Midget and Sprite Technical index

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