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MG Midget and Sprite Technical - Copper vs Kunifer brake pipes.

When I refreshed our Frog I used a Copper brake pipe set from Moss and decided to use Silicon Brake fluid as well.

The whole of the brake setup was replaced with new, calipers and cylinders both slave and master, discs and drums.

Ever since the first test drive the brake pedal has felt soft. Not soft in the way that you have to pump the brakes to stop, but in the way that when you press the pedal it doesn't feel solid.

I've always thought it could be the Silicaon fluid but I had the thought that maybe the real culprit is the give in the soft copper brake pipes. Maybe I should use Steel or Kunifer next time?

What do you think?

Rob aka MG Moneypit

I prefer to use cunifer/kunifer, but I doubt that the copper pipe is the source of your soft pedal.

I had *heard* that silicon fluid can give you a spongier pedal, but have no direct experience of it.
Dave O'Neill 2

If you used rubber hoses then perhaps a set of Goodridge hoses might firm up the feel.

But if you've only recently renewed the brake set up then perhaps the brakes just need more use 'to settle' - when my wife's car got new discs and pads it took a few days of me driving the car to get the pedal feel back to how it was.
Nigel Atkins

When bleeding brakes l always do it twice, with a shake-down drive between. Or better, a few day's use as Nigel says. All the microscopic air bubbles get shaken out and coalesce and can then be bled out properly on the second session.
Guy Weller

I should have put I drove my wife's car further (and harder) than she would have been able to with her commitments at the time to speed the process up.
Nigel Atkins

Well it's been a couple of years since and it went to Le Mans and back this July and it is still the same. I think Dave O's description of Spongier is a better description so maybe it is the fluid.

How hard is it to bend cunifer pipe compared to copper, which I managed by hand?

Rob aka MG Moneypit

Rob. I think it is the fluid, but if you want to fit cunifer,I have a pipe bending tool you can borrow.
I've heard so many pros and cons about silicon fluid that it's hard to form an opinion without me using it. And as it requires changing all the seals, (I think), it's not going to happen. I'm quite happy with Dot4.
Bernie Higginson

Not sure about the truth of this but I always understood that fluids are incompressible, in which case Silicon and normal brake fluid should both behave the same.

What would make a difference if either of them is more prone to containing air bubbles, gasses being compressible.

Rob, how about the security of your pedal mountings? If there is any looseness in the pedal or m/c mounts or flexing of the top of the footwell, that could feel like a spongy brake pedal.
Guy Weller

I will give my five peneth on Silicon brake fluid.

I have used Silicon on all of my three cars the longest since the early 1990's.

My daily driver is a 1990 XR3i. In the many years of Silicon I have changed one brake cylinder. I have owned the car from new.

I also have a Frogeye 948 bored 0.125 with 5 spd still on A40 drums fitted by P.O. That I have owned 47 years. Travelled extensively throughout Europe, Czech Republic, & Hungary.

Then I have a 1330 5 spd Sebring Rep.that I built in 2013.

There is a slight springiness in the pedal noticeable when first converting. I no longer notice it.

It is recommended to change rubbers when first changing to Silicon but I didnt - so twenty years later or thereabouts.
All the cars have Goodridge flexible hoses.
I didnt wash the systems out beforehand either!

All the cars are used. You can read of my last 1800 mile tour in the Sebring rep at

There is so much c**p spoken of Silicon Brake fluid.


Alan Anstead

Another vote for silicone. Doesn't take the paint off when you're bleeding the clutch either :). It supposedly doesn't absorb moisture either so shouldn't require changing as frequently as traditional brake fluid.
I used it when restoring the car 8 years ago and still have the same fluid in the system. The clutch master cylinder seals failed last year but I suspect that was due to it being purchased many years ago (and even then was NOS), rather than the fluid.
J Tickle


I've just re-activated a thread about my home made pipe bending tool.

Large radius bends are quite easy to do by hand with cunifer, but the tool was invaluable for tight bends.

It's a bit harder to bend than copper, but not as bad as steel.
Dave O'Neill 2

Copper pipe for brake lines is a no go here
If you turn up for a rego inspection with copper pipes it's a --NO

I think it was something to do with not being able to visually check if the pipework was proper hardened copper tubing or soft stuff, so they banned it full stop
William Revit

Well I also thought Silicon brake fluid didn't attck paint but it does as I found out when doing the Frog. Maybe because it was freshly applied acrylic but I was swapping out a copper pipe to the clutch cylinder for a flexi pipe (concentric slave with type 9 gb). I left it unattended while I answered the phone and it dribbled out and down the inner wing strengthening triangle. I put a piece of kitchen towel down to soak it up and forgot about it.

Next day the kitchen towel was embedded in the fresh paint. The paint had softened and it was a PITA to remove it.

I also had an occasion (on the same car) where another distraction (easily distracted me) let fluid drip down the outside of the inner wing and down the sill. I didn't notice it so some time later when I finally twigged (silicon drips on the garage floor) I toweled it down and it left a discoloured streak on the sill which I can't polish out.

So silicon fluid does effect freshly applied acrylic but doesnt have the severe stripping action of dot4.

Willy. I'm not sure what we buy here. I do know there is something printed down the length of the pipe. Maybe we should take note of what it says?

Rob aka MG Moneypit

"Maybe we should take note of what it says?" What and break the habit of a lifetime?
You'll be suggesting reading instruction manuals next :)

The paint had been on for a couple of years and was earlyish 2 pack so was quite hard before the first spill! Wiped off more or less straightaway and no marks to show. Have you tried detailing clay to remove the stain Rob?
J Tickle

Why don't modern car manufacturers use silicon brake fluid? I assume they don't because they still suggest changing the fluid every couple of years because of the hygroscopic nature of the fluid (which would be an advantage of silicon as it doesn't pick up moisture).
Graeme Williams

"Why don't modern car manufacturers use silicon brake fluid?"
and why don't any racers use it either?

because it gives a spongy pedal and reduces 'feel'
David Smith

Funny that people say it gives a spongy pedal! Most other MGA owners comment on how good my (standard) brakes are compared to their own cars, and I have never used anything other than silicon fluid.

You DO still need to flush with silicon fluid, as any water that gets in collects at the lowest point of the cylinder and over time eats into the wall. That's why 100% cleanliness and dryness is also important on assembly.

But unlike normal brake fluid, moisture isn't absorbed into the silicon fluid so it doesn't allow moisture and rust to spread across the whole system as it absorbs water from the surroundings. The silicon stuff also doesn't cause corrosion where there are any leaks or weeps. And it doesn't strip paint that has hardened.

I suspect that many problems that people experience are down to their brake components being beyond their serviceable life, and that is certainly a common finding when I get asked to sort out poor braking - original hard (formerly flex) hoses, weeping rusty cylinders, and seals that are way past being flexible, and brake shoes that are either oily or contaminated with brake fluid. I still use normal fluid when maintaining other people's cars, but the silicon is staying in mine. The midget I did two years ago was built with a complete new system, so that too has silicon fluid, and the brakes are GREAT.

So I find when a system is properly bled and maintained, silicon fluid has a LOT of advantages, but one significant disadvantage - the extra cost. Over 30 years of using it, I only see the advantages and a lot of false "facts" that are really just hearsay. In financial terms it pays for itself easily.
Dominic Clancy

Normal fluid is hygroscopic - it actively attracts water moisture which is absorbed by the fluid which means it cannot be bled out (unless you change the fluid) When under pressure this water "boils" creating water vapour, which being a vapour is compressible, hence the spongy pedal. The boiling point of water at the high pressures in a braking system (up to 400psi)is well below that of the brake fluid. The advantage is that when his water is bound in with the fluid it cannot collect in droplets within the system and is less prone to cause rusting.

Silicon fluid is not hygroscopic so it doesn't attract water, is less likely to suffer from the boiling issue so it doesn't create a compressible water vapour in the system. However, water may still get into the system in which case it doesn't mix but stays in droplets which can coalesce and cause rusting. Which is why it is wise to change silicon fluid regularly - though probably not very frequently.

Both systems can get air bubbles which need bleeding off, and these may not necessarily be ejected through the pipes and out of a bleed nipple very easily. Hence the advice to bleed, drive to shake them through the system, then bleed again.

Under normal conditions both Dot 4 and silicon fluid, both being liquids, should not be compressible and should behave the same as far as "spongy pedal syndrome" But they do behave differently in the way they attract and absorb water and it is the tendency for this to boil to a compressible gas (water vapour) that causes the problem.
Guy W

extra cost? for a road car maybe, but not compared to decent competition-spec fluids. Having tried various other brands and still had boiling fluid / low pedal and other woes I now use Castrol React SRF which is 50 a litre - but no problems however hard the brakes are used.
David Smith

This thread was discussed between 03/12/2016 and 04/12/2016

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