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MG MGB Technical - DOT 3, 4 or 5 Hydraulic Fluid

MGBs were designed to use DOT 3 hydraulic fluid in their brake and clutch operating systems.

This fluid, in addition to working the brakes or clutch, had the other properties of being poisonous, stripping cellulose paint and absorbing water from the atmousphere - ie it was hygroscopic.

Many people have changed to DOT 4 fluid, because it has a higher boiling point. I understand that DOT 3 and DOT 4 fluids are compatible and DOT 4 can simply be added to a system already containing DOT 3.

However, despite it's non-hygroscopic nature and its lack of paint-stripping characteristics, there seems to be concern about DOT 5 fluid in some circles, particularly regarding 'compressibility'. Can anybody answer the following questions?

a. Can DOT 5 be used in earlier systems without purging the system and changing all the seals?

b.
Peter Hills

Oops, sorry, didn't complete the questions but the computer took my text away!

b. Can DOT 5 fluid be left in systems without the bienniel drain and replace considered appropriate for DOT 3 and DOT 4?

c.
Peter Hills

Sorry again - the tab key sends partially completed text - I'll try again!

c. Is the 'compressibility' of DOT 5 a problem?
Peter Hills

Lot of info in the archives on this one. Take a couple hours and read up.
.

Peter-

a) The seals won't need changing, but the system should be purged and cleaned with denatured alcohol as the two are not really compatable.

b) Yes.

c) Yes, and so is low lubricity Pistons in slave cylinders and calipers have been know to stick, leaving the car with locked-up brakes.

There is also a question that you didn't ask: d) If I switch to DOT 5 Silicone Fluid, can I change back to DOT 4 or DOT 3?

d) Only if you completely clean the system of all of the DOT 5. To date I know of nothing that will do this. I tried puting up a posting on this BBS to find out if anyone knew of anything that would dissolve it. I came up blank. Once you switch to DOT 5, you're stuck with it.
Steve S.

I did change back to DOT 4 from silicone fluid but as Steve said I'm not sure I got all the silicone
fluid out of the system, First I drained the complete brake system and flushed it four times with
denatured alcohol. I rebuilt the calipers and disassembled and cleaned the almost new rear
cylinders. I did not disassemble the master cylinder. The brakes are working ok but I'm not out
of the woods yet. Prior to the change the brake pedal was lower than it should be. The pedal is
still lower than I like after lots of brake bleeding and several adjustments. It is firm but low and I
suspect I'll be replacing the master cylinder soon. FWIW, Clifton
Clifton Gordon

I use the new dot 5.1 fluid
Ross

Peter-
I forgot to mention that switching to DOT 5 silicone brake fluid just because it is less harmful to paint is defective logic. If your master cylinder is leaking, then you're risking both your life and your car by driving it in that condition. Rebuild the master cylinder so that it won't leak and it won't matter whether or not the brake fluid has the potential to damage the paint. The best brake fluid I've seen is Valvolene Synpower. It's compatible with both DOT 3 and DOT 4 and has a boiling point of F555, which is a good 271 degrees more than DOT3 (which boils at a mere F284). As such it's got the one great performance virtue of DOT 5 and none of its vices.
Steve S.

OK Ross, what is 5.1???

I did mention on this BBS sometime back the apparently preposterous notion that a solvent intended to remove silicones from surfaces to be painted MIGHT be capable of also removing it from brake systems. I too asked a question as to whether there might be a chemistry guru who might shed some light on this phenomena and if it were truly possible given proper care and procedures or just another empty guess. I never heard anything from anyone either.

If there are any chemical engineers or chemists out there reading these postings, I think a whole automotive community would be delighted to know the answers to the compatibility issues revolving around using or not using silicone based and conventional glycol based brake fluids. I would think that these questions come up often enough to warrant some explanation. But in today's litigous atmosphere, perhaps no one is willing, even under a (god help us!) pseudonym, to give out such information?? Sorry to be so catty about this but after these many years, you would think that the info we seek could be had in a plain and simple and authorative form and not the frustrated guessings and musings of enthusiasts.

If YOU know where to find it, please share, as it would sure save a lot of rummaging through bias and misunderstandings.
Bob Muenchausen

And then there's the issue of *spilling* the brake fluid on the paint. I really hate to say it, but it seems that every stinkin time I mess with the stuff it somehow manages to get on some paint somewhere. Last time it was a MC replacement. First, it's just about impossible to get every bit of fluid out before removing the cylinder. Then with the new one in and starting to bleed I learned I hadn't tightened the banjo bolt. I know, my own fault, but it's always something. So with synpower now running into the carpet, it does eat paint btw. Now if your MC starts to leak isn't it just so lovely to not only get it on your shoes, but to get to take the entire works apart for new paint again as well? Sorry, I'd say that is indeed a valid reason for silicon fluid. Now, what's the story on this 5.1?
Jim Blackwood

Did a quick search and found:
http://www.vtr.org/maintain/brake-fluids.html
and,
http://www.commaoil.co.uk/bp.htm

Wayne
Wayne Pearson

Guys, bear with me a moment. Aren't we supposed to shy away from all Dot 3 stuff? Isn't this the suff that eats away all natural rubber or is there a different Dot 3 in Europe I don't know about?

BTW, I use Girling Dot 4. Never had any problems except when I have to top up the clutch resovoir. Royal PITA. Never fails to spill a little somewhere.

Luis
Luis

OOps, make that Lockheed brake fluid. I just checked the bottle in my trunk. :)
LUIS

Regarding DOT 5.1, this is covered in Wayne's reference
http://www.vtr.org/maintain/brake-fluids.html

Quite a useful discusion - at least it admits to the existence of DOT 5.1!

No idea of how accuratye the information is but it seems reasonable.


Peter Hills

On the assumption that Silicone Fluid is not patented, why do car mfts and brake companies not use it or sell it?

For show cars it may be an advantage but I would not trust it in a performance car.

Paul
Paul

Paul-
Silicone brake fluid is intended for use in the braking systems in large tractor-trailers (I believe that they're called "Lorries" in the UK.) and some aircraft brake systems. These braking systems are different from the ones used in automobiles. Here in the USA a container of silicone brake fluid is sometimes marketed with a disclaimer on the can: "Not For Use In Automobile Brake Systems." Unfortunately, as yet there is no regulation requiring that this warning be on all containers of silicone brake fluid, but some marketers put it on the can only as a CYA for liability issues. Caveat Emptor!
Steve S.

I also use the Vavloline Synpower and feel it is the best on the market. There are advantages to silicone brake fluid but the real question is what is the need. The minimal requirement to change conventional fluid on a regular basis doesn't seem to justify the effort. People live with vehicles that never have this task accomplished. That is not to condone such behavior.

If one is racing or driving extremely hard then the brake system will be open on regular occations. If not, then I would think looking at the brakes every couple of years is reasonable.

Many of the MG's are not driven on a daily basis. A vehicle that is used on an infrequent basis need just as much maintenance as a daily performer. Just let the rest of your vehicle go for a while and you will find a hard lesson.

Use the best conventional brake fluid you can find and be careful adding the stuff. Use fender covers. I am sure we can all find places to spend the money that is saved.
Bob Patzwald

I haven't monitored this board for about a year now. It's interesting to see that this battle is still raging. I've used DOT5 in my 1965 B since it's restoration in 1990. I replaced the brake light switch in 96 or 97. Other than that, it had caused zero problems. The car is driven regularly (1000 miles a year). Low maintenance and no water absorbtion are among the reasons I chose to change.

My Harley Davidsons (and most other motorcycles regardless of country of origin) have had DOT5 from the factory since 1978 that I know of.

If I had it to do over, I would go with DOT 4; but I see no reason to revert now.
Bud

And I think it will continue to rage. Probably because all needs are not equal. For instance, my brakes are so understressed that I would probably never get them hot enough to boil out moisture or air from the fluid. http://www.foresight.cc/Graphics_Content/MG/Front_Brake.JPG Certainly they have never been spongy. I've had silicone fluid in the systems (separate front/rear) for close to 20 years now. However, wear and aging of seals has been an issue, as I have replaced the front calipers and both master cylinders, though not the rear cylinders that I recall. OTOH, I just installed one of those horrendously expensive HTOBs and although the spare seals sat in silicone fluid for 6 months with no signs of deterioration, I put Synpower in the system because it felt slippier and I do not care to have the HTOB wear out. Evidently stainless bore liners are the next step.

For people who don't mind messing with the brakes the dot 3 and 4 are just fine, but for me it rates right up there with tailpipes, rust repair and gear grease among my least favorite tasks. I hate the feel of brake fluid, don't especially care for the smell of it, and when mixed with brake dust I find the whole ordeal rather disagreeable. For those fellows who like to flush out their brake fluid between heats at the races, more power to you. It ain't happening here.
Jim Blackwood

Castrol LMA DOT 4 is compatible with all systems, and is not hygroscopic like DOT 3. I haven't tried the syntheic mentioned, but have gotten very good use and no moisture problems with Castrol LMA DOT 4 (my cars are store in an unheated wooden barn in N.American winters).
Ira Spector

This thread was discussed between 24/04/2002 and 29/04/2002

MG MGB Technical index

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