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MG MGB Technical - Fluid Leak?

Walked out into the garage and found fluid around my passenger rear wheel. Pretty sure it is brake fluid ( I run Dot 5 silicon). What is odd is that the car has been sitting for 3 months while I rebuild the motor. Have not sat in it or pressed on the brakes.

No big deal to pull the wheel and hub and take a look, but I have not seen a brake leak from just sitting.

Any thoughts?
Bruce Cunha

It tends to be a more common problem when using Silicon braking fluid. Standard DOT fluids contain additives that swell up the seals inside your braking system. Silicon fluid does not and over time can allow the seals to shrink, especially if the vehicle isn't used on a regular basis. RAY
rjm RAY

Ray, Another good reason never to use silicone. Even Girling don't like the stuff!
Allan Reeling

Nor Lockheed:

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"A comment by Brian Smart, Service Technical, AP Lockheed:

Our technical service department is receiving an alarming number of calls from motorists reporting problems with silicone fluids. AP Lockheed neither markets such fluids nor recommends their use with our own or any other braking system.

Virtually all the problems relate to long/spongy pedal, sudden loss of brakes and hanging on of brakes. They reflect certain properties of silicone fluids identified by us over many years and recently ratified in SAE publications, namely high ambient viscosity, high air absorption, high compressibility, low lubricity and immiscibility with water.

Research has shown that the relationships between problems reported and properties identified may be expressed as follows:

Long/spongy pedal

Compressibility, up to three times that of glycol based fluids.
High viscosity, twice that of glycol based fluids, leading to slow rates of fill and retention of free air entrapped during filling and hence bleeding difficulties.
Sudden loss of brakes
Air absorption - gasification of absorbed air at relatively low temperature produces vapour lock effect.
Immiscibility (failure to mix) with water - whilst the presence of dissolved water will reduce the boiling point of glycol based fluids, any free water entrapped in silicone filled systems will boil and produce vapour lock at much lower temperatures (100C or thereabouts).
Hanging on of brakes
Low lubricity - in disc brake systems the sole mechanism for normalisation if system pressure upon release of pedal pressure is a designed-in tendency of seals to recover to their "at rest" attitude. Low lubricity works against this tendency.
High viscosity, exacerbating the above effect.
It should not be assumed, therefore, that the high price of silicone fluids implies higher performance in hard driving or even normal road use.
AP Lockheed glycol based fluids do not contain the adverse properties described above. The recently introduced Supreme DOT 5.1, which exceeds the performance criteria of DOT5, is suitable for all conditions likely to be encountered in modern driving conditions."

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And once you have silicone, you are pretty-well stuck with it.

The thing that I cannot get my head around is that Whilst DOT4 is an improvement on DOT3 and both are glycol-based, and DOT5 is silicone i.e. a completely different fluid to and incompatible with DOT 3 and DOT4, in their wisdom the powers that be chose to call the improvement to DOT4, DOT5.1, which makes it sound like a development of DOT5! That's why in the UK at least DOT5.1 is often referred to and labelled as 'Super DOT4'.
Paul Hunt

Well yes, I've read all that stuff about silicon fluid too. But then it's published by a manufacturer of glycol fluid, so they are hardly going to say it's the best thing are they!

All I have to go on is my own experience. I have had silicon fluid in the clutch and brakes of my V8 for at least seven years and 21,000 miles. I don't have a spongy pedal: I don't have a long pedal travel: I don't have the brakes hanging on: I've never had problems with the brakes on long descents when they have got really hot: and I don't have to change the fluid every couple of years to prevent it from corroding the pistons! In fact my brakes feel and operate perfectly normally.

A friend had silicon fluid in the brakes of his factory V8 MGB GT for 17 years before he sold it. During those years he had to do absolutely no maintenance on the hydraulics.

OK, OK, if it's so good why doesn't any manufacturer recommend it? I don't know. All I know is that it has worked for me (and my pal).
Mike Howlett

Gents,

In 1985 I restored a 1957 Ford Retreactible Hard Top. A BIG steel top convertible. All new brake system throughout and filled with silicone fluid. Eventually got a offer I could not refuse and sold to an acquaintance a few mile away. Recently stopped by for a visit and when he raised the bonnet I noted the tag I had put on the master cylinder saying use silicone fluid. Here we are 30 years later and he says he has never even adjusted the brakes. Of course it is only driven to shows and in parades.

Regards,

Jim Haskins
J. M. Haskins

Down under we can't buy silicon from brake suppliers. Apparently some rubbers used in the seals are incompatible with the silicon.

Herb
Herb Adler

You never see it on ABS equipped vehicles due to the fact that it compresses and throws off the sensors going to the main computer that control the braking system. It certainly has its place in some hydraulic systems and many people swear by it, but it does come with some unique problems. RAY
rjm RAY

It's the low lubricity of silicone that prevents its use in ABS systems, as it is the fluid that has to do the job of lubrication of moving parts in the system.

Silicone fluid does need changing, and as often as glycol. The same amount of moisture gets into the brake system regardless of the fluid, especially on MGB systems where the breather hole in the cap is a direct passage and not a diaphragm. The moisture will distribute itself through glycol, but lie in 'lumps' in silicone, neither being a Good Thing. It can be harder to flush out from silicone systems as it lies in the bottom of calipers and clutch slaves i.e. away from the bleed hole. At least brake slave cylinders have the bleed hole at the bottom.
Paul Hunt

This thread was discussed between 21/02/2015 and 22/02/2015

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