"soft" brake becomes "hard" again

[Dieter H. Marschall]

Hello everybody,

During a weekend trip in our TR6 I realized a strange phenomenon with our brakes: After some hard driving in the mountains and about 35° C ambient temperatures the brakes became softer and softer and the tracel of the brake pedal longer. The fluid in the small, front compartment of the master brake cylinder reservoir was dark with some black slime. So the seals in the rear wheel cylinders were obviously dissolving.

I was already contemplating a complete brake overhaul and possibly a swop to silicone brake fluid...

The next morning during some temperate driving the brakes were "hard" and the way of the brake pedal short again.

Can anybody please explain this phenomenon to me?

Kind regards,
Dieter - from Germany.

[jleyton]

Hi Dieter,

I'm not sure that this will be the explanation, but here goes:

As you may know, conventional brake fluid is hygroscopic - that is, it absorbs water. Both being liquids, they are incompressible and so, in normal usage, a system where the brake fluid has apsorbed water will function ok.

However, water has a much lower boiling temperature than brake fluid and, in spirited driving (or more correctly, braking) the water will begin to boil and cavitate. I don't claim to understand the chemistry, but in doing this, the water effectively releases air which IS compressible - hence a soft pedal.

Another problem of water in brake fluid is that it will lead to corrosion of metal brake components.

When brake fluid absorbs water it darkens in colour so, just poissibly, you may only need to change the liquid, but I'd definitely check the seals.

Cheers

Julian

[Seaking]

Dieter
if you change to silicon brake fluid I would suggest that you renewal all seals inc master cyclinder.
the reason behind this is I used silicon fluid and the master cyclinder seals failed. No other seals have failed as new brake cyclinders where used inc pdw valve.

cheers Glenn

[GDPR]

Agree with young Julian here, and would like to add that it is very unlikely that detritus from deteriorating rear wheel cylinder seals would travel that far back up to deposit in the reservoir - more common with seals from front calipers to do it as there is a shorter pipe run, plus if you just retract the pistons and return the fluid to the master cylinder when changing pads instead of releasing the bleed nipple then you get the cooked and discoloured fluid back up.

Personally I think your master cylinder is on borrowed time, and would be careful to limit your pedal travel when changing the fluid or use a pressure bleeder in case there is corrosion further down the bore that will ruin the seals in the master cylinder. Stick with standard type fluid until all the seals are changed.

Russ

[jleyton]

"Young" Julian - thank you Russ, although, for the first time ever, I have some doubt in your judgment

Cheers

Julian

[GDPR]

You have more energy than some kids I know Julian, hence the deserved "young" title...

[Dieter H. Marschall]

Brakes

Agree with young Julian here, and would like to add that it is very unlikely that detritus from deteriorating rear wheel cylinder seals would travel that far back up to deposit in the reservoir - more common with seals from front calipers to do it as there is a shorter pipe run, plus if you just retract the pistons and return the fluid to the master cylinder when changing pads instead of releasing the bleed nipple then you get the cooked and discoloured fluid back up.

Personally I think your master cylinder is on borrowed time, and would be careful to limit your pedal travel when changing the fluid or use a pressure bleeder in case there is corrosion further down the bore that will ruin the seals in the master cylinder. Stick with standard type fluid until all the seals are changed.

Russ

Hello,

I think I will renew the master and the slaves cylinders and change to silicone then. Keep fingers crossed....
Maybe there is too much water in the brake system by now - although I renewed the DOT 4 fluid two years ago. Perhaps this explains the fact that the fluid in the front reservoir turned dark and the pedal travel became longer when hot. Over night the brakes and the fluid cooled down again and the problem was "solved".

Thank you all for your advice.

Kind regards,
Dieter.

[Stagnant]

Hi Dieter, I would say Julian is correct.
The ratio of expansion from water to steam is something like 1 : 1600 (if I remember my fire college days !) so a very small amount of absorbed water becomes alot of 'air' in the brake fluid when they're worked hard and the water boils.

Cheers
Mark.

[wilf]

I can't go along with the change to a silicon fluid. It is a "race" item, designed to be changed every other lap! Well ok, Race. Using it long term in a road car is a very different situation.

The issue with silicon fluid is this - it may not be hygroscopic, but water can and does still get into the system in the long term. As it cannot "disperse" through the fluid, it clumps, and you can get worse brake fade due to "clumps" of water boiling than if you had normal Dot 4 and simply changed it every 2 years. Which is a much better way of going about things IMHO.

And all of this is totally separate to having to change every seal in the system before you put the silicon fluid in.

For a Stag, the max operating temperature of "ordinary" DOT 4.1 fluid is more than sufficient. Unless your surname is Hart.

[jakesmig]

I've used silicon fluid in my Midget for 15 years and the Stag for 7 without adverse effect.

The Mg is virtually a track day car and the brakes, which are heavily modified, tended to suffer long travel on both DOT4 & 5 but not so much on silicon. They'll do 20 laps round Knockhill without showing any serious deficiency.

[TR5convalescent]

Silicone fluid is fine as a clean install in a new system. It is not hydroscopic but if water gets into the system it will normally gather in the calipers which is not where you want it it. You need to be absolutely sure the system is sealed and if installing in an existing system it needs to be pressure flushed to get all the old fluid out. Some advocate flushing with meths. It is also probably a good idea to change all rubber components including flexible hoses.

If you are planning to re paint the engine bay do that first as when your master cylinder eventually leaks, as they all do, you will never be able to paint the engine bay again.