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MG MGB GT V8 Factory Originals Technical - Silicone Brake Fluid

With regard to Silicone Fluid I've set out below a warning re using this fluid.

Roger Parker sorted my problem out but this means flushing out the silicone fluid and replacing all seals and replacing with AP Lockheed recommended types (not a fun job).

Our technical service department is receiving an alarming number of calls from motorists reporting problems wi~
silicon fluids.
Virtually all of the problems relate to:
Long/spongy pedal
Sudden loss of brakes
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
immiscibility with water
Research has shown that the relationships between problems reported and properties identified may be
expressed as follows:
Long/spon gy pedal
(a) compressibility, up to three times that of glycol based fluids.
(b) 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.
Sud den loss of br akes
(a) Air absorption. Gasification of absorbed air at relatively low temperature produces vapour lock effect.
(b) 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).
Hang ing on of brak es
(a) Low lubricity. In disc brake systems the sole mechanism for normalisation if system pressure upon release o1
pedal pressure is a designed-in tendency of seals to recover to their 'at rest' attitude. Low lubricity works
against this tendency.
(b) High viscosity, exacerbating the effect of (a) above.
It should not be assumed, therefore, that the high price of silicon fluids implies higher performance in hard drivinc
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 DOT 5, is suitable for all conditions likely to be
encountered in modem driving conditions.

Paul Wiley

All of the warning statments are true. We found this out about 15 years ago when it became popular in the states. It was thought to be the cure for water absorption in all of those classic cars that spent most of thier time in storage.
My first trouble with it was the master cylinder pistons not returning all the way. (poor lubrication) This would not allow the m/c to refill and of course resulted in poor braking.
Any change of elevation would bring out air bubbles like getting the bends.
Needless to say, use the recomended fluid and change it every year.


I have used silicon brake fluid in my GT V8, for five years without any problems.

The car is regularly used on Continental rallies, particularly in the alps, and the brakes (4 pot callipers & ventilated disks) are excellent.

The only problem is that they squeal!

Nigel Steward

I used the silicone fluid for some years in my TR4 and got tired of the spongy pedal feel. I replaced it with Castrol LMA and the pedal feel is much better.

BTW, I got the same spongy pedal with a Subaru Legacy. The only benefit I can see to this stuff is it doesn't take off paint.
Bob Cline

I use silicon brake fluid in my Roadster V8 for approx 3 years. I replaced all seals and hoses and don't have the spongy feal and never had problems with the brakes.
I also use it for the clutch.
Peter van de Velde

Four years with silicone so far. Car sees a couple of track days a year. Its the first time I have ever had any paint on the pedal side of the engine compartment. Its also the only time I HAVE had a firm pedal in over 20 years of ownership. Go figure...

Pete 67gt
Peter Plouf

Please note that AP/Lockheed will NOT honor warranties on their remote type power brake servo (as used on Home Market Bs and V-8s) if silicone fluid has been used in them. Also, Lockheed pressure type brake light switches seem to fail prematurely when exposed to silicone fluid. The only solution is to remove the switch and flush it with carb cleaner, then reinstall and bleed the system. This is a task that I don't relish doing every three months!

Paul Kile
Paul Kile

I have used silicone for YEARS with no problems in my MGB's. My good friend Bob Thompson in NC also had no problems for years.......until recently. He has had the same problem that you describe. I have relaxed my grip on the broken bottle that I carried to the top of the rampart years ago. Thanks for your contributions.

David Gable

Article in Novembers MG Enthusiast about a just-completed rebuild in 1993 that suffered brake failure has AP Lockheed stating that silicone fluids are not compatible with the seals used in MGB M/Cs - they swell and can cause the brakes to jam on. Maybe those people that have not experienced problems have different seals somehow.

Paul Hunt

Paul H, good point - my guess is different source factories worldwide use different materials which is why some have problems when other sdo not).
Anorak note - when i visited PK earlier this year he was conducting long-term soak tests with both types of fluid. PK, are the jars still on top of your fridgr and what's the conclusion ?
David Smith


Yes, the jars are still atop the fridge. For those of you who are interested, in September of 1999, I placed two identical sets of Lockheed rubber seals (clutch seals and dust boots for early MGB) into two 50ml sample jars. I added Castrol GT LMA DOT 4 fluid to one jar, and standard American DOT 3 (CarQuest brand) fluid to the other.

I looked at them this weekend and there is no visible difference in seal size. I think I will remove the seals here soon, dry them off, and check size with my dial calipers.

When I performed this test about 20 years ago (only with one set of seals and one jar of DOT 3 fluid) I saw a dramatic swelling and deformation of the seals after about a week of exposure to the fluid. After several months, the seals had decomposed into a rubbery sludge at the bottom of the container.

There may be several reasons why I did not get a dramatic result this time around. Lockheed may have gone to synthetic rubber in their seals, or the formulation of DOT 3 brake fluid may have changed.

In any case, I will measure the seals here soon, and report the results. Note that this test did not include the use of Silicone (DOT 5) fluid.

Paul Kile

PS: Got the engine and trans installed in the V-8 bodyshell on Saturday morning - spent the rest of the day putting all the ancillaries on. Now all I need is to detail the radiator, and the power unit installation will be complete. Progress!!!
Paul Kile

This thread was discussed between 10/10/2000 and 30/10/2000

MG MGB GT V8 Factory Originals Technical index

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