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MG Midget and Sprite Technical - Leaking caliper

I obtained some S/H calipers a few years ago as part of the Frogeye rebuild and conversion to front discs. I cleaned them up and fitted new pistons (not stainless) and seals and put them to one side. They were fitted last year and silicon fluid added to system a few weeks later and system bled. Car still not on road but today I noticed a bit of (expensive) brake fluid on the garage floor. One of the refurbished calipers is leaking. Either a piston is rusty or scratched or I nicked the seal, presumably?

Honestly, is it worth stripping and rebuilding again trying to extract pistons and get that awkward dust seal in? I can get a caliper from Bhive for £48 and swop it straight in. Don't know if it has stainless pistons. What to do?
Bill Bretherton

I recall reading on here that some of the replacement calipers had a problem with leakage at the joint due to poor manufacturing.

Can you check where the leak is on yours. may just be from the union.
Bob Beaumont

One criticism I read of silicon fluid is that it doesn't lubricate the rubber parts as well as the mineral oil type and seals tend to harden faster when not in use and seal less well.
GuyW

Bob, union is dry, I can definitely feel fluid at base of caliper below inboard piston.

Guy, that is a worry. Maybe they've been standing for too long - is that bad? Maybe I'll swop it for a new caliper just before going on road.

I've never had a leaking caliper on other cars, including those with sticky calipers that I've had to free off.
Bill Bretherton

Silicon Fluid is not a problem i've used it in both my 'B' rebuild and my midget and that hasn't had the fluid changed since I built it over 8 years ago + Silicon is a good lubricant.
Unlikely stainless pistons for that price.

Bill its no problem getting the pistons out, just take them off the car, cramp one side from moving and just a footpump will blow the other side out.

I'm sure I've got some old calipers 'in store' you are welcome to if you want to use them. your existing pistons should still be good - so only seals required. I have not found it necessary to split the calipers when rebuilding.
I used red grease when I rebuilt the 'B' calipers.

R.
richard b

ETA: I was typing as Richard was posting.

I've not read that but wonder how it can tie in with the idea of silicone fluid being used for military vehicles that would stand in storage for years.

I've read on this BBS someone who accidently mixed silicone with glycol and only noticed when he saw the different colours on changing/bleeding much later and he said he had absolutely no problems from the mix.

And someone else who just swapped silicone by draining the glycol as the silicone filled. I keep meaning to check with a mate who put silicone in his rebuilt MG 20 years ago and that only really gets summer use and IIRC he hasn't changed it in all that time, and he put it in his other MG many years ago, that vehicle is used regularly and no brake problems.
Nigel Atkins

Just something I read in a comparison table between Silicon and mineral hydraulic oil. I think the point was that mineral oil causes the rubber seals to swell with time and this improoves their ability to seal securely. I suspect it is marginal though,

But I do wonder if it is partly due to them having stood with fluid in, but not pressurised? The seal is shaped such that it's the pressure that causes the lip of the seal to be pushed against the piston. If fluid is there, but not under pressure might it begin to seep past the lip after a prolonged period?

You could try pushing the pistons back a bit, clear of the disk, and then press the brake pedal hard a few times and see if this 'sets' the seals any better. It would be easy to try, and nothing lost if it doesn't work.
GuyW

Bill if you go down the new route, try these people. We use them for a lot of race car components and their service is very good and prices usually very competitive.
https://www.powertrackbrakes.co.uk/caliper.html
David Smith

Richard, thanks for the offer.

David, thanks, I've heard of powertrack and believe them to be good.

Guy, I'll try that, be good if it worked! The brakes are bled but haven't been used in anger yet.
Bill Bretherton

Guy,

Do you have the article to hand to verify the details. IIRC Citroen were the only common maker to use a mineral hydraulic oil for the brake system on cars like the DS and later. The common brake fluids are glycol based if not silicone https://en.wikipedia.org/wiki/Brake_fluid .
David Billington

No David. I realise I have been using the term mineral when I probably mean glycol based. It was just stuff I read when I was trying to decide what fluid to use in my car. I decided against silicon.
GuyW

I did a quick search for 'lubricity silicone brake fluid' and came up with some useful info

just a couple.

https://www.calverst.com/technical-info/brakes-silicone-brake-fluid/

kewengineering weird page layout, scroll down for info

https://www.kewengineering.co.uk/Auto_oils/brake_fluids.htm

I'll address it again when the car is nearer back on the road but likely will stay with glycol based, it always worked for me before.
David Billington

David,

I think this link is an updated version of what you posted giving Automecs views.

Apart from the American military ISTR Citroen and Rolls have been mentioned as users.

This subject comes up as regular as wheel bearing and oil S***ing - I just use it and as I don't do track use (or need special pads for track use either) I just forget about the problems of changing fluid ever 2 years and rusty wheel cylinders etc and have never found an issue with pedal travel etc etc.

https://www.mginfo.co.uk/upgrades4mgs/Fluids/brake_clutch_fluids.html

R.
richard b

Same as Richard. New system installed in 2008 and the silicone fluid is still in there.
The only secondhand parts were the calipers and they were long stored NOS bought in the late 90's from a chap in Leeds who was rationalizing his fleet of classics :) So far so good; not going to say more as Fate is hiding behind the garage door just waiting to be tempted.
Jeremy MkIII

It is always interesting to hear about silicon brake fluid.

It wont work. It wont work with this-n-that.
Only recently I was told that it cant be used on a TR6 because it was fitted with a servo?

My modern, daily driver, is a 1990 Ford Escort XR3i. Mk4 EF1.
Discs at front drums at rear.
It has a servo.
I bought it new. It has now done 229,000 miles.

I has had silicone fluid since mid 90s. Still has original calipers although the rear slaves have been changed in the past couple of years.

The silicon brake fluid performs better than the camshafts which are the Achilles Heal of the hydraulic tappet equipped CVH engine. Currently on number four.

Both of my Sprites are longtime users of silicon fluid.

Alan Anstead

TBH, after much thought I decided marginally in favour of silicone for my Frog, and intended flushing through my 1971 car system and switching to silicone for that too. So I bought 5 litres. Only after getting it did I discover that DOT5.1 isn't, as one might reasonably expect, an enhanced form of DOT 5. So I had a gylcol not a silicone fluid after all. And quite a lot of it!

Compensation is that 5.1 has a pretty good specification, plus it simplifies the job on the 1971 car as I can just drain and refill without possibly needing to change seals, or at the least having to flush through with meths or the like. Both cars now have fresh fluid and I still have half of it left over.
GuyW

So much simpler if Dot 5.1 had been allocated some other figure to avoid confusion.
Alan Anstead

Hi Bill,

When I restored mine recently I found a difference between the two kits I bought. The first one fitted really well the other was way out. I couldn’t even get the outer metal retainer to fit and seals where loose. So I sourced a replacement.

The replacement fitted much better but was still not quite as tight as the first one that went in so well, but I thought it was good enough. The pistons fitted really well and moved slowly with decent resistance.

I haven’t tested them yet so no idea if mine will leak, but thought I’d share these differences.

https://www.mgmidgetrestoration.com/2021/06/front-brakes-rebuild-and-fitting.html

I’ll update you when I get to that stage. Might be some months mind you! 😉

Kind regards

James
James Paul

Thanks James. I was in the Bristol Moss today so got one of their seal kits to try. It's just one of the four pistons leaking. Typical!
Bill Bretherton

Hi Bill,

That’s where I got the poor kit from. The one from MGB Hive was the third one that fitted better.

Good luck

James
James Paul

Whilst not Lockheed probably better than the latest rubber quality ?

https://www.ebay.co.uk/itm/313476304894
richard b

James, read your post after I got back so too late! I'll see how the fit is. I refurbished the calipers 7 years ago with seals from Sussex classic car parts via eBay. Pistons from MEV spares.

Richard, thanks but I'd worry about NOS rubber - won't it have deteriorated?
Bill Bretherton

I worry about new rubber !!!

Do you yet know why you had a leak ?

If its still not fixed with your new seals I'll have a 'look see' for the calipers in the garage.

R.
richard b

Thanks Richard. Not done it yet, out today and tomorrow.
Bill Bretherton

NOS rubber would have to be very old and poorly kept to be worse than the piss poor rubbish rubber of the modern variety.

I've got some steering rack gaiters to replace those I put on 4 years ago, and the TRE boots have lasted an amazing 7 years unless I've misfiled a replacement. Previous modern made piss poor rubbish rubber steering rack gaiters lasted only months of light use (all in the Archives I'd guess).

I'd not put any money on the longevity of of modern made piss poor rubbish rubber.
Nigel Atkins

Should be able to get genuine Lockheed (now ‘920 Engineering’, was until recently ‘AP Caparo’) caliper seal kits from Powertrack Brakes.

Thanks
Mike
M Wood

A wee comment about bleeding caliper brakes. I had real problems getting a firm pedal after rebuilding the calipers on my TF 135. Eventually I was advised to push the pistons back and hold them there with a G clamp. Hey presto an excellent pedal in minutes. Apparently if the pistons have been removed there is a large air bubble behind them. As you attempt to bleed fluid passes above the bubble but does not force it out. This is especially so if the fluid inlet and bleed nipple are close to each other and at the bottom of the caliper.

Cheers

Jan T
J Targosz

This is the leaking side of the caliper and the piston. Some rust as you can see - caliper refurbished 7 years ago and stored dry. I can feel the seal edge all the way round as if it's correctly inserted. Other half and other caliper not leaking but may have rust spots. But is it time to replace the caliper?

Note it's the first time I've done this so I may have made mistakes/ lack of brake grease for storage etc. I value any advice. I realise my life depends on properly rebuilt brakes.





Bill Bretherton

Bill

Another option is get them rebuilt by company like Past Parts - https://www.pastparts.co.uk/index.php?route=product/category&path=215_150
or Coasting Limited: http://www.coastings.co.uk/

The rebuild costs look they might be good value and would give you piece of mind. If your calipers are beyond rebuild try both these companies, as well as Powertrack, for new good quality replacement.

Best wishes
Mike

M Wood

I’d take the seals out, clean up the rust with fine wire wool or wet and dry paper. Thoroughly clean, replace with new seals and apply the red calliper grease.

Piston looks ok. Should be quite tight when fitting.

Shouldn’t cost you more than £15.

Try that first before sending them off buy new ones.

James Paul

Thanks Mike, that's an option.

James, I might rebuild it - it's just the cost of silicon fluid if it leaks again! And that damn dust seal retaining ring is a pain. I remember doing them originally on a freezing winter's day when I could hardly feel my fingers so I may well have snagged the fluid seal. It was quite a bad leak meaning the m/c had dropped too much so I'll have to bleed all the brakes again. Oh joy!
Bill Bretherton

Bill

The piston should be really smooth with no rust spots - as its what is in contact with the seals.
If its at all pitted I would replace it and assemble with the rubber grease.

R.
richard b

Richard

The piston isn't pitted as such, just some light rust spots which should clean up. But, of course, something has caused it to leak. I'll have a go at cleaning it up. I'd rather use a refurbished Lockheed casting than a new Chinese one.
Bill Bretherton

Hi Bill,

If the retaining ring is in good condition I wouldn't bother trying to replace it. Mine were rusted so had to really.

One thing I noticed when working on mine that you might like to try, is fit a new outer seal. If you can move the seal in any direction then see if you can press the retaining ring in a little so the seal is in there nice and tight.

Your retainer ring does seem to be sitting a little higher than mine does.

I used a vice and piece of chipboard to press mine in.

Also at about 4 o'clock / bottom left on the picture of the outer seal it looks like is has gone in on itself. Is just the picture or is the seal equally over the lip of the retaining ring all the way around?

Thanks

James



James Paul

James

Dust seal was quite well aligned so left it but attempted to push retainer in a bit more. I removed fluid seal, cleaned up the caliper, especially the fluid seal groove then cleaned up piston and re-fitted with new seal, using brake grease. Couldn't really see why it had leaked. Caliper back on now with new copper washers on hose (I find Sprite calipers awkward to align with backplate, hose retainer and locking plate). I'll bleed tomorrow and see if it holds for the next few days before tightening caliper properly.

Thanks for all the suggestions. I did ring Bhive who are offering calipers for £48 but they're Chinese. Told me they didn't have any core units to refurbish so well worth hanging on to old Lockheed calipers.
Bill Bretherton

This thread was discussed between 14/06/2021 and 21/06/2021

MG Midget and Sprite Technical index

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