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MG MGB Technical - Synthetic Brake Fluid?

I'm pretty sure this has been around before but I couldn't find it in the archives.

My problem, which I'm about to cure is that the old brake light switch has died (again, this time quickly!) so I've ordered a replacement, hopefully fit this weekend. Has anyone got any thoughts about swapping the brake fluid over to synthetic? Will this increase the longevity of the switch? I'm still toying with the idea of fitting a relay next to the switch as discussed on a previous thread.

Any ideas or should I just bung in the new switch???

Thanks
David Cross

I think it has as much to do with the crap quality of the switches, my later B has the mechanical switch and I'm on my third one in 5years.
I've used silicone fluid for nearly 10 years and had no problems with it, only thing is you need to completely clean out the old stuff and renew all the seals before you change, it was easy in my case as I was rebuilding the brake system with new lines, flexes etc. whether its worth the hassle if your not rebuilding is another matter.
Ron
R. Algie

Opinion was at one time that changing to synthetic was *causing* switch failures. But others have said there is no fluid near the contacts and it is poor quality reproduction replacement switches that are the problem. Either way, changing to synthetic isn't going to increase the longevity of the switch, so if you have to change the switch again (how long has the present one been in use?) and you cannot get the OE item you could well benefit from fitting a relay. If so, connect a diode across the relay winding such that it *opposes* normal current flow. If you get it round the wrong way your new switch will probably only work once!
Paul Hunt

That then begs the next question... where do you pick up your good quality repro gear?
David

David,

Depends on what you mean by Synthetic?

Castrol SRF is a synthetic racing brake fluid.

Silicone based brake fluids are non hygroscopic, unfortunately silicone based fluids are compressible. They produce a soft pedal which is OK unless driving in a spirited manner and do not care about precise modulation, and are not planning on using their car at altitude.

Paul
Paul Wiley

Paul,
Why altitude? I am using silicone in Denver, CO (5,280 to 14,200 in the mountains) and haven't noticed signficant difference when I changed. Have bled them out several times so no air left in system, though.

JTB

J.T. Bamford

Silicon fluid has a poor bulk modulus compared to normal DOT 4/5.1 diester based brake fluids, which by definition in the lubricants business, are also synthetic as Paul mentions.

Positives of silicon include the non-hygroscopic, as well as a high temperature capability and non-paint destruction when spilt on the bodywork! But, an engineer from Molykote told me that the reason the fluids never took off with the OEM's is that the design of the systems and the master/slave size ratios had to be different for the best performance and they weren't willing to go that road. Moreover, in controlled ABS/EBD systems on modern cars this introduces complexities for the fluid.
Whilst the manufacturer of the silicon fluid I use quotes compatability with normal fluids (not mineral oil brake systems - as on some RR & Citroen models), they do reommend replacing the entire fluid asap if this cross mixing has occurred.
As said above the best time to try silicon is after a system rebuild with fresh seals and pipes etc. Tag the reservoirs clearly so that any mechanic working on the car knows!

Normal fluids, apart from stripping paint, do absorb moisture which leads to rust and corrosion issues, along with a lowering of the boiling point thus inducing loss of compression when the brakes get hot. DOT 4 is fine for modern cars, except when towing a caravan and working the brakes hard in Summer! Racing cars use DOT 5 (Sil) or DOT 5.1 which are rated for higher temperatures as you'd expect in hard, repetitive braking into corners. This repeated overheating of the fluid can destroy it, and anyone that does track day sessions with their car should think of more frequent fluid changes to avoid fluid failure.

FWIW
Martin ZT

Paul,

"unfortunately silicone based fluids are compressible"

I was taught that fluids are generally considered "incompressible" from most engineering perspectives, although there was a small factor to consider under extreme circumstances. Do you have an idea of the compressibility ratios normal brake fluids vs silicone? I'm considering changing mine over. Thx,
Edd Weninger

Edd, see the following article of brake fluid and compressibility. The entire series are a good read.
Regards,
Dennis
http://www.buckeyetriumphs.org/technical/Brakes/Fluid/Fluid.htm
D F Sexton

David - To answer the question as to where to pick up a good reproduction brake light switch, the simple answer is - you don't! Unless there has been a great improvement (and I mean GREAT improvement) over the past couple of years, the replacement switches are c**p, which is why I went with the relay circuit. The replacement switches do not use contacts as such, just two very light pieces of spring bronze impinging on each other. That is why the relay and the arc supression circuit have become necessary. Cheers - Dave
David DuBois

Somewhere back in the archives, may be on the A board, there is a thread that covers using a Chevy or similar brake light switch. It requires making up a bracket but the end result apparently is a switch that is nearly indestructible and also relatively inexpensive to buy. If anyone is really interested contact me off line and I will search my archive files at home.

Larry
49 YT
58 A
69 Cs in restoration
Larry Hallanger

Martin,
Not sure I follow this point
master/slave size ratios
Must be a big difference in fluids, Silicone with more air, as most who do use it do not believe it is a problem.

ABS is interesting in those air bubbles can effect this system but they are robust when adding larger brake discs?
Another interesting article?
http://www.minispares.com/ms/BRAKES___Silicone_brake_fluid__to_use_or_not_to_use___that_is_the_question__785.htm

Paul
Paul

Should also add the brake system is not a sealed system any water that enters the system will sink to lowest point, unlike normal brake fluids that absorb water and go off slowly (race a bit quicker), the free water will boil at 100deg C with possible vapour lock.

Paul
Paul

Paul is right, swater in silicone systems go to the lowest part and the fluid must bebled once every few years - like 3 years or so.
Mike MaGee

I am sure that it is true that any water that gets in the system with silicon fluid will migrate to the lowest point, but I have to believe that this is a non problem. One would have to really work at it to get water in the brake system. Since the silicon fluid is not hygroscopic, it doesn't draw moisture out of the air as DOT 3 or 4 does and the vent in the cap does not easily transmit water, even under pressure, intot he master cylinder, so one would have to remove the master cylinder cap and pour water in. I have had silicon fluid in the brake system of our TD for the past 20 years and have never once bled the system to get rid of any water in it. To this day the fluid remains crystal clear, with no sign of corrosion anywhere in the system. This with a master cylinder that is mounted under the driver side floorboard where it is exposed to the full spray of rain water on the roads of Western Washington State where we drive the car year around regardless of the weather. Cheers - Dave
David DuBois

Notes from Castrol SRF advise which is formulated differently from DOT 5.0

We do not recommend using DOT 5 fluid in any racing applications. DOT 5 fluid is not hygroscopic, so as moisture enters the system, it is not absorbed by the fluid, and results in beads of moisture moving through the brake line, collecting in the calipers . It is not uncommon to have caliper temperatures exceed 200 F, and at 212 F, this collected moisture will boil causing vapor lock and system failure. Additionally, DOT 5 fluid is highly compressible due to aeration and foaming under normal braking conditions, providing a spongy brake feel. DOT 5 fluid is best suited for show car applications where its anti-corrosion and paint friendly characteristics are important.

Paul

Paul

"where do you pick up your good quality repro gear"

The terms 'good quality' and 'repro' are mutually exclusive.

Whilst the hydraulic switch can be replaced with the later type of mechanical switch with a bit of work the repro versions of both are probably as bad as each other.
Paul Hunt

Paul S. _ I can only comment on my own observations after having used silicone fluid for some 20 years in our TD and 7 or 8 years in our MGB. I have not observed the problems of moisture collection and vapor lock nor have I experienced a problem with compressibility of the fluid in the brake system of either car (although I did have a compressibility problem, probably due to areation, in the clutch hydraulics of the MGB and have subsequently gone back to DOT 4 for that application). I did have a lubricity problem, causing scuffing of the primary seal in the master cylinder of the MGB that is probably due to the reduced lubricity of silicon fluid. If I have the problem again with the rebuilt cylinder, I will probably consider changing back to DOT 4 fluid for the MGB. The other known problem with silicon fluid is that flow through very small orficies, such as found in ABS braking systems and with self adjusting balancers will cause areation of the fluid and in those systems, silicon should not be used. I have found, however in our cars that the silicon fluid works quite well and completely eliminates any corosion in the system. I am always suspicious of statements like found in the Castrol document when they are made by companies that do not produce or sell silicon fluid and tend to dismiss them as rather self serving. Similiar statements have been made by AP Lockheed regarding silicon fluid (they also do not make or sell silicon fluids).

I believe that the bottom line is the information available on the Department of Transportation which sets the standards for all brake fluids used in passenger cars and the Federal Motor Vehicle Safety Standards, Standard No. 116; Motor vehicle brake fluids. The FMVSS can be viewed in it's entirity at: http://frwebgate.access.gpo.gov/cgi-bin/get-cfr.cgi?TITLE=49&PART=571&SECTION=116&TYPE=TEXT
If silicon fluid was as terible as Castrol and AP Lockheed have implied, it would have come off of the market years ago.
Cheers - Dave
David DuBois

It's my understanding that the United States Postal Service was the original advocate for DOT 5; I believe that it is used in their delivery vehicles - lower maintenance.

rn
RN Lipow

RN - Silicon fluid was developed for the military for their motor pools where vehicles are kept in storage for long periods of time. The silicon fluid,not being hygroascopic, keep the brake system from developing internal corrosion (sounds like good stuff for the special interest car group who only drive their cars on sunny Sunday afternoons in the summer doesn't it). It would not suprise me if the USPS went to it to reduce maintenance on their vehicle brakes, particularly after the experience I have had with the stuff. Cheers - Dave
David DuBois

I am guessing here, but I am led to believe that there must be several formulations of "synthetic" brake fluid besides silicone. The stuff I use is the Valvoline SynPower Dot 4 which, in my experience seems to embody the good qualities of "regular" glycol based brake fluids (lack of compressibility, easier bleeding) and the less corrosive qualities of silicone fluids when spilled on paint. I get an excellent hard pedal with this stuff, it lasts without problems and discoloration for about 2 yrs, and when spilled, will wipe off if attended to promptly. If I were to let it sit, however, it will eventually attack the paint. This is at least less scary than the old style fluids that seemed to be about the best and quickest paint remover ever invented. FWIW
Bob Muenchausen

Bob - "synthetic fluids are DOT 5.1 (Damned computer designations are taking over everything) and is not a silicon fluid. I have not delved into what it's composition is, but from what I have heard and read so far is is diferent formulation of glycol fluid. By the way, the old glycol fluids, besides being a very effective paint remover is also an excellent penetrating fluid for stuck bolts and such. Cheers - Dave
David DuBois

David,
I would agree with your comments and most of the DOT 5 problems can be overcome for a general use low maintenace fluid. I would question its use in hard driving race situations and ABS, although if the US Postal Service drive the way our UK post office drive their vans that may be debateable.

In praticable term DOT.5 is 3 times as expensive but if you change say every Six years then cost wise there is nothing in it. Most owners change to copper and SS items so rusting is not really an issue. Glycol will attack paint so you need to be careful but this applies to DOT 5 to ensure air is not in system.

However with DOT 5 if you have a braking problem you can never be sure it is not the fluid, and if it is not fluid, you could use a lot of expensive fluid to find the problem ?

SRF is a Silicon ester so Castrol have the technology?, which is interesting if you consider the move to Synthetic oil low maintenance extended drains, then Silicon fluid would be similar for brake systems, so why are Castrol not exploiting the market?

Paul
Paul

Bob,

Synthetic as a term can cover many things giving rise to court cases over definition of oil products.

Synpower brake fluid is based on Polyglycol ethers and esters?

Paul
Paul

Paul, agreed, synthetics is a loose term to imply man-made, but, ref. your point of legality, Group III hydro-cracked mineral oils are legally termed synthetics in the US. They perform with similar levels to synthetics owing to the highly refined and pure nature. Group I and II mineral oils are more likely to contain impurities (sulphurs and waxes) and a more random mix of molecules, hence their problems with oxidation and other degradation mechanisms which affect their performance and service life.
Martin
Martin ZT

Paul S. - You are correct regarding the use of silicon fluid for racing - it is not done. Also, our mail delivery driveres over here do not emulate yours in the speed of deliveries. In fact, I believe that the one that delivers on our route has a chalk mrk made next to his foot each day so they can determine if he has died during his shift. Cheers - Dave
David DuBois

Oh, here we go again! The old "Silicone Vs. Everything Else" chestnut. I'll weigh in, if only out of a sense of duty. Amongst the brake fluids presently available there are three possible candidates. The first, DOT 3, is a poor choice for high performance driving due to its low wet boiling point of F284 (dry boiling point F401) and is now generally considered to be obsolete. Be advised the American DOT 3 brake fluid is formulated differently than British DOT 3 and is incompatible with the Original Equipment natural rubber seals used throughout the brake system. It will slowly but surely dissolve them! The second candidate, DOT 4, is much better with a wet boiling point of (dry boiling point F446). Of the different Brands of DOT 4 brake fluid on the market today, Castrol LMA appears to be the best. The third choice is a synthetic fluid, Valvolene SynPower, which wins with a wet boiling point of F343 (dry boiling point of F513). DOT 5 Silicone-based brake fluid is a poor choice for any automobile. SAE publications have ratified the symptoms of its poor performance by identifying certain properties prevalent in silicone brake fluids. These have been named as high ambient viscosity, high air absorption, high compressibility, low lubricity, and immiscibility with water (failure to mix with water). It would seem that there are definite relationships between these properties and symptoms. Its compressibility is up to three times that of the more common glycol-based types, so needs more pedal travel to actuate. Its viscosity is twice that of the glycol-based fluid - meaning it is thicker. This equates to slow fill rates that can trap air, making bleeding of the brake system a real bear. Entrapped air suffers gasification at relatively low temperatures, causing a vapour-lock effect. Should you decide to use silicone-based brake fluid, be sure that all of the seals are in excellent condition as it will easily find its way past a leaky seal and air will get into the system. In disc brake systems, the sole mechanism for returning the pistons to their normal ?not in use? position away from the disc is the tendency of the seals to recover to their 'at rest' attitude once the pedal is released. The low lubricity of the silicone-based fluid works against this recovery; it?s high viscosity amplifying this effect, sometimes allowing the pistons of calipers and/or slave cylinders to bind in their bores and lock up the wheels. Its immiscibility causes any free water caught in the system to boil at relatively low temperatures - producing a vapour-lock. Glycol-based fluids absorb water (the water dissolves into the fluid), and although this will reduce the boiling point, it is unlikely to cause severe vapour-lock at low temperatures. With a boiling point of F500, its performance is inferior to that of Valvolene SynPower. While it is true that silicone-based brake fluid does not absorb water, water still gets into the system through condensation. Because water is heavier than silicone fluid, it will ultimately sink and gather in the lowest point in the system. Should it freeze, line blockage and brake failure becomes possible. Be sure to flush the system with denatured alcohol prior to refilling it with the silicone fluid. Failure to do so will result in the residual glycol-based fluid interacting with the silicone fluid to form a sludge which will destroy the seals in the system, resulting in catastrophic brake failure.
Steve S.

Steve,

DOT 4 Fully Synthetic Fluids usually are around 600f dry 399f wet slightly better than Synpower at 513f

Wilwood Super high temp is 620F and cost wise same as DOT 5

Difficult info to find is how hygroscopic are these fluids and therefore how often you need to change to give cost benefit and performace.

Paul
Paul

Out of technical ignorance, I have taken to flushing my brake systems (regardless of what they held) about every 2 yrs. The reward for this maintenance has been apparent long system seal life and relatively clear fluid removed with each flush. No, it is not as clean as new fluid, but you can see through the jar in most cases as more of a tint than a cloud of debris. FWIW

Thanks also for the explanation of synthetic. I figured it was one of those words that ad copy generators played fast and loose with and was also dependent upon trade laws for definition. ;-)
Bob Muenchausen

This thread was discussed between 06/02/2004 and 10/02/2004

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