sticking brakes

[2cams70]

I can see we just aren't going to come to any agreement on this one Rohan. I know of no OEM brake caliper supplier that has ever used stainless steel pistons in their calipers. If they were a great idea the OEMS would have taken it up by now with no issue. This despite the fact that stainless steel pistons would be cheaper to manufacture not having to undergo any plating process. Steel pistons have been in use and proven since the introduction of disc brakes in the 1950's. Stainless steel pistons are a relatively recent introduction into the market (by the aftermarket not OEM) and mainly on low mileage hobby vehicles not the ones doing the hard grind of daily use. I for one won't be replacing my pistons with stainless steel unless there's an absolute need and let others do the experimentation for me. Lots of stainless steel pistons available on Chinese Alibaba website site - go for it guys!!

Wasn't that long ago when silicone brake fluid was being promoted as the greatest thing ever to use in your brake system. Well it isn't and no way would an OEM ever use that stuff!

[RichardHawkins]

Corrosion is a tricky subject with lots of detail differences that can make a huge difference. I have spent all my professional career in the chemical process industry, and have observed surprising results with materials that should and shouldn’t work. As a group of chemists and engineers using sulphuric and hydrochloride acids we thought we had a reasonable grip on the subject, but had learnt that the only real answer was to try and test materials in the actual environment.

We needed to design and build a process using nitric acid, what a mess! As one of my colleagues commented “step one mm outside your area of expertise and you can be in deep shit very quickly” and we were.

I don’t know why chrome plated steel is used, but I think it is also used on the hydraulic cylinders in earth moving equipment. Sometimes people have something that works and see no reason to change when they have other much more pressing problems.

Richard Hawkins

[StressCraxx]

They cause rust because they promote corrosion of the cylinder the piston is in. The iron cylinder corrodes preferentially to the stainless steel piston. Hence you get a jammed piston unless you take extra precautions to keep water out of the area under the dust boot.

Not sure I agree they "promote corrosion". Stainless steel is passivated due to the protective layer that forms, but i don't think it forms an active galvanic cell that promotes cast iron corrosion in the callipers. Using zinc plated or high zinc primer coating on the callipers will help protect the cast iron by sacrificial corrosion of the exterior coating

My original Elan callipers still have their original zinc plating / galvanizing ? Not sure how they were originally done but life in Australia much easier compared to UK from a corrosion perspective.

cheers
Rohan

I'm in industrial plant engineering as well, particularly mechanical seals with dynamic and static elastomer seals. Some parts sources sell stainless pistons thinking they will not corrode from moisture. Unfortunately, they will. The chrome plating on the steel pistons is hard chrome plating, the same used on hydraulic piston rods. It is corrosion and wear resistant as long as the surface remains polished, hard and passive with a good chrome oxide layer.

Brakes are hydraulic systems using glycol based fluids. The glycol is hygroscopic and by nature absorbs moisture. The hard chromed surfaces are corrosion resistant up to a point. It's pure chromium, highly corrosion resistant and very hard, usually Rockwell 65-70C. Eventually the chrome layer wears away from a process called fretting corrosion, eventual loss of the passive oxide layer. Once the hard chrome surface is gone, the corrosion accelerates with the steel.

Stainless pistons seem like a great idea until one realizes the stainless has less corrosion resistance than hard chrome plating. The stainless pistons are 18% chrome, 8% nickel and the rest is iron. The stainless pistons are inherently softer, Rockwell 50C or less than hard chrome and more subject to fretting corrosion because the oxide layer is more fragile.

I have used both stainless and OEM hard chromed steel pistons. The hard chrome plated steel lives far longer.
YMMV

[rgh0]

I can see we just aren't going to come to any agreement on this one Rohan. I know of no OEM brake caliper supplier that has ever used stainless steel pistons in their calipers. If they were a great idea the OEMS would have taken it up by now with no issue. This despite the fact that stainless steel pistons would be cheaper to manufacture not having to undergo any plating process. Steel pistons have been in use and proven since the introduction of disc brakes in the 1950's. Stainless steel pistons are a relatively recent introduction into the market (by the aftermarket not OEM) and mainly on low mileage hobby vehicles not the ones doing the hard grind of daily use. I for one won't be replacing my pistons with stainless steel unless there's an absolute need and let others do the experimentation for me. Lots of stainless steel pistons available on Chinese Alibaba website site - go for it guys!!

Wasn't that long ago when silicone brake fluid was being promoted as the greatest thing ever to use in your brake system. Well it isn't and no way would an OEM ever use that stuff!

I actually agree with much of what you're saying

OEMs use hard chrome plated pistons probably due to their increased wear resistance and reduced risk of leakage due to the very hard polished surface. They tend to ultimately fail due to corrosion of the underlying steel through microcracks in the chrome creating a galvanic cell and that occurs well after the warranty period !

In a low mileage classic using stainless steel pistons does not appear to give any major wear problems despite being softer than hard chrome plated. Personally I have been using them in my Elan since 1980 and my Plus 2 since 1986 without any wear or corrosion issues so I have done some experimentation for you

I also experimented with Silicone brake fluid when it first came out but I did not like the spongy pedal feel due to the greater compressibility and changed it out after a couple of months. I also doubt the claim that it reduces corrosion as its not hydroscopic, that just means that water that gets in past the piston seals sits at the bottom of the callipers and happily corrodes away rather than being absorbed into the bulk of the fluid and flushed out with regular fluid changes.

cheers
Rohan

[patrics]

Hi,
Stainless pistons were tested by Girling / Lucas but they had there own problems, some of which were:
Inconsistent rollback
Poor seal nip so the piston moved back into the housing easier by comparison to chrome - already a problem with all opposed piston calipers especially with the flexible suspension arrangement.
The pistons tended to wear quickly by comparison to chrome.

At the end of the day it is the housing which corrodes first causing the piston to seize. I use plastic pistons in the front of my car and still have the same issues.

Regards
Steve

[rgh0]

Hi,
Stainless pistons were tested by Girling / Lucas but they had there own problems, some of which were:
Inconsistent rollback
Poor seal nip so the piston moved back into the housing easier by comparison to chrome - already a problem with all opposed piston calipers especially with the flexible suspension arrangement.
The pistons tended to wear quickly by comparison to chrome.

At the end of the day it is the housing which corrodes first causing the piston to seize. I use plastic pistons in the front of my car and still have the same issues.

Regards
Steve

Hi Steve
The differences in surface finish possible and hardness would account for most of the observed differences and why OEMs (and hydraulic rod component makers) have stuck with hard chrome on carbon steel.

What fails first probably depends on the environment but in Australia my pistons failed due to corrosion well before significant calliper corrosion.

Never heard of plastic pistons ???

cheers
Rohan

[patrics]

Hi Rohan,
For sure it will depend on the environment but there can’t be to many calipers with original plating left so with low use age that area quickly becomes problematic.
The chrome on the original pistons was always rubbish as there was no copper base coat. It is probably possible to get pistons with copper base layer but who knows what the quality is like dimensionally and obviously if the plating is done right then the piston won’t be round.
Plastic pistons should really termed as phenolic pistons which is something Lucas developed in the 80’s

Cheers
Steve

[peter Battrick]

I had the same had problem. The brakes only released when I lifted the pedal with my foot. Only solved when I ment mentioned i to a mini afficionado. He said it was a common problem. Usually caused by a sticking master cylinder. Changed my refurbished master cylinder for a new one and bingo, problem solved.
Moral of the story is not to bother replacing seals on brake components yourself. Replace with new or send away to Classic Autocars for a proper job. As a testimonial they have done an excellent job on my Elan brakes and my '84 Porsche 911.

[2cams70]

I had the same had problem. The brakes only released when I lifted the pedal with my foot. Only solved when I ment mentioned i to a mini afficionado. He said it was a common problem. Usually caused by a sticking master cylinder. Changed my refurbished master cylinder for a new one and bingo, problem solved.
Moral of the story is not to bother replacing seals on brake components yourself. Replace with new or send away to Classic Autocars for a proper job. As a testimonial they have done an excellent job on my Elan brakes and my '84 Porsche 911.

If the cylinder bore and piston of the original master cylinder is in good condition there should be no reason to replace it. In fact the cylinder if original is likely to be of far better quality and to original tolerances compared to a reproduction from Asia. The master cylinder in these cars is a pretty basic device. It should be fairly simple for any reasonably competent person to replace the seals.

[peter Battrick]

I agree with everything that you say but, on an almost 60 year old master cylinder, wear and corrosion take their toll on original equipment that was manufactured down to a price and not up to a quality. If the old cylinder or piston are worn then don't waste time trying to replace seals, just instal a new unit. Life is too short.

[2cams70]

I agree with everything that you say but, on an almost 60 year old master cylinder, wear and corrosion take their toll on original equipment that was manufactured down to a price and not up to a quality. If the old cylinder or piston are worn then don't waste time trying to replace seals, just instal a new unit. Life is too short.

You say you agree with me but then go on to say exactly the opposite! Corrosion mostly effects cast iron cylinders not aluminium ones and you really are talking rubbish if you think original Girling parts are not quality.

[nmauduit]

I had the same had problem. The brakes only released when I lifted the pedal with my foot. Only solved when I ment mentioned i to a mini afficionado. He said it was a common problem. Usually caused by a sticking master cylinder. Changed my refurbished master cylinder for a new one and bingo, problem solved.
Moral of the story is not to bother replacing seals on brake components yourself. Replace with new or send away ....

well, I would not be quite that drastic, I believe it is certainly feasible by any enthusiast to overhaul a master cylinder, one needs to source proper parts and use his or her best judgment to proceed though, then the result is a lot more rewarding than bolting on new parts, more often than not of dubious provenance and specs...