sticking brakes

[street]

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

[street]

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

[69S4]

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?

[2cams70]

Yes stainless pistons actually cause rust.

[69S4]

Yes stainless pistons actually cause rust.

I'm hopping from foot to foot expectantly ...

[2cams70]

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.

[69S4]

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.

[rgh0]

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

[2cams70]

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

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

https://www.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.

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?

I gave some suggestions in this previous thread

viewtopic.php?f=42&t=49256

[69S4]

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?

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.

[2cams70]

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 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.

https://msdspds.castrol.com/bpglis/Fusi ... Grease.pdf

[Billmack]

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

[Donels]

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.

[2cams70]

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

[rgh0]

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.

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