Sticking brakes

After a year lay-up all running again (engine rebuild), I refurbed rear brakes and master brake servo, replaced brake servo. On running definitely feels like i have binding and after braking feels like brakes take 1-2 seconds to release.

Any suggestions

i was reading some of the other posts and there is quite a bit about pushrod length, this made me think that my dad said to me some time ago the only way they got the brakes to work was to add washers to master cylinder mounting. I didn’t really pay much attention, however this seems to be the likely culprit.

I changed to a willwood MC and changed pushrod to original lucas keeping the length, installed with no washers, so i think i will add some washers to move pushrod and see what happens

I’m in the middle of refurbing my front brakes as the pistons are sticking on one side. That gave me pretty much the symptoms you describe. Check they’re not sticking because they’re gummed up first before ripping into the rest of the hydraulics. I’m surprised to find they’re tight as they’re stainless pistons. I’d have expected to find the usual chrome plated ones rusting but dirt seems to be the culprit this time. Anyone else had problems with stainless replacements?

Yes stainless pistons actually cause rust.

I’m hopping from foot to foot expectantly … :laughing:

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.

You mean the ‘lip’ area in front of the seal (not the body of the bore behind the seal where you’d only expect there to be brake fluid)? What ‘extra protection’ under the dust cover would be sensible?

The original pistons used to rust all along their length (although mainly in front of the seal) - presumably from water in the brake fluid, although that’s only a guess.

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

So what are your thoughts on this article Rohan particularly given your background in plant engineering:

appmfg.com/blog/how-to-prev … less-steel

I’d suggest there’s some pretty good reasons why OEMs don’t use stainless steel as a piston material and this is just one of them. It’s certainly not cost because stainless is pretty cheap these days.

I gave some suggestions in this previous thread

viewtopic.php?f=42&t=49256

Ok, that’s pretty much the conclusion I’d come to and the course of action I’d taken - up to the point of packing grease under the dust cover. I stopped at that point, somewhat concerned that the grease may not stay in there when the calliper gets hot. I guess you don’t have the same concerns. The front callipers are in bits on the bench at the moment so the opportunity presents itself.

I use Castrol rubber grease here and on the piston prior to insertion in it’s bore. Seems to work well. I can understand your concern though. I haven’t had any issues with grease run out however during normal on-road usage. I’d think the dust seals would be pretty much toast very soon anyway at temperatures 150 degrees + that this grease seems capable of withstanding.

msdspds.castrol.com/bpglis/Fusi … Grease.pdf

Watch for bad flex hoses also. They cause a similar problem by swelling up inside and acting like a low grade check valve

Hmmm. I’m not sure I agree with some of the science here. Yes stainless steel does promote corrosion, in some circumstances. My last home was built during a copper shortage in the early 70’s and had stainless steel piping. It did cause corrosion at the brass fittings where the pipes joined the copper hot water cylinder, resulting in the zinc being used up in the fittings resulting in cast copper fittings, which could break fairly disastrously. These required replacing every 10 years or so.

But back to the the callipers. The stainless pistons are not in direct contact with the cylinders as they are slide on the rubber seals, so no corrosion should occur on the inside, unless there is a lot of water in the brake fluid. There should be no electrical contact path to promote galvanic corrosion.

Externally, in my experience the pistons seize because of a ring of corrosion under the dust cover preventing them sliding through the seals. Stainless steel pistons should therefore prevent this and should be better.

In the presence of water as a conductor stainless steel is lower than cast iron in the galvanic table with a value of around -0.3 compared to -0.5 for cast iron, whereas mild steel is higher than austenitic cast iron in the table with a value of -0.65 compared to -0.5. So stainless steel will promote slightly more corrosion of the calipers, BUT mild steel pistons will corrode more than cast iron calipers. It is corrosion of the pistons that causes the brakes to seize so stainless steel pistons should always be best.

When the inside surface of the cylinders rust the rusts expands and creates friction against the pistons so it’s not just corrosion of the pistons only that causes sticking but rather corrosion of either or both

Yes the difference in galvanic potential of stainless steels of various types and carbon steels or cast irons of various types exists but it is relatively small and while it can be a problem in some environments ( e,g, continuous sea water exposure) it appears not to be a problem in most brake piston applications in practice for multiple reasons as pointed out. Zinc plating or painting with inorganic zinc primer will further protect the callipers as was originally done.

Stainless steel should be no worse than the original chrome plated carbon steel pistons in terms of galvanic action with the calipers in any case

cheers
Rohan

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!

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

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

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 :slight_smile:

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

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