View Full Version : DOT 4 vs DOT 5

12-18-2002, 12:25 PM
For a winter project, I am rebuilding clutch master cyl and slave (master cyl fluid was black). I am also replacing rubber brake lines in the rear of my 74 TR6.
Now would be the time to change over to DOT 5 if I am ever going to do so(I would also rebuild the brake master cly and rear wheel cyl if I changed over). I have read about the differences on other list, but I would like to hear your opinions on the subject.
I put about 5000 mi a year on my 6. It is in storage of only 3 months a year.

12-18-2002, 01:31 PM
Haven't gotten around to the brakes yet but when we put the rebuilt engine back in my 7 we also rebuilt the clutch slave and master cylinders and went with the silicone fluid. Can't remember the specifics as to why, but it seemed to be the way to go from all that we'd read about it on this and other places. Seems to be working out fine at 6000 miles since 4/21/02. Will surely use the silicone fluid when we do the brakes. graemlins/yesnod.gif

12-18-2002, 01:36 PM
I am all so interested as I am completely redoing the brakes on my TR3. I understand from another post on this fourm that the dot 5 is the way to go if you completely redo a system. It doesn't corrode the parts as bad as the Dot 4. The down side is some people say the pedal has a little sponger feel than the Dot 4. It also boils easier which should only effect you if you race or live in the mountains. All this information comes from another post of this forum.

12-19-2002, 08:42 AM
Hello Carmine,
I agree with Dale on this, I have used Silicone brake fluid for about twenty years and would use nothing else on a road car.
It does not degrade with age (It does not absorb water as conventional mineral fluid does) and also does not corrode the cylinders, particularly useful if the car is stored. It does not lift paint when spilt on the body but it is quite a bit more expensive to buy. Worth it though in my opinion.

Alec graemlins/cheers.gif

12-21-2002, 07:34 PM
Carmine , the real problem is the natural rubber seals that are the heart(and achillies heal) of the system. Castrol LMS and silicone are the only safe options.Every other conventional fluid degredes the seals in short order, leading to sudden failure. I sugest the silicone for normal uses, the LMA for more serious driving.Of course if some brite lad came up with modern seals to re-build our LBC's brakes life would be much simpler
MD(mad dog)

01-20-2003, 08:20 PM
WOW... didn't know silicone fluid was around that long??? Main thing for me was the fact that it doesn't EAT PAINT!!!!!!!!!!!!!

01-21-2003, 05:36 PM
I re-did my clutch with silicon back in 1987 and never had trouble with it. I heard something bad about using it brake lines but I do not remember what it was. Propably not that important.

01-21-2003, 05:45 PM
I understand it (DOT5, although I'm not sure about 5.1) isn't desireable in ABS systems. Unless we're talking about new/modern vehicles this isn't an issue.

There are also other negative comments regarding moisture absorption; since silicons don't absorb moisture, any moisture left in the system will be allowed to pool and concentrate causing localized corrosion and eventual failure in those areas. If you're rebuilding an entire system this shouldn't be a factor, and if you change the fluid out and clean the system I would think the moisture would evacuate itself.

Trevor Triumph
01-30-2003, 12:58 AM
I changed to DOT 5 in my early Spitfire several years ago for the reasons stated... no threat to the paint, and less corrosion. I have noticed that the silicon seems to be thinner than other brake fluid and hence leaks around some seals. Is that possible? The left hand rotor and caliper has a residue of brake fluid on it and the car pulls to the right when I apply the brake. I clean the rotor and pads and we're stopping straight again for a while. I don't think film of fluid is left over from bleeding the system. T.T.

01-30-2003, 02:47 PM
I'm not sure if DOT5 is thinner, but I've read conflicting stories about replacing DOT3/4 with DOT5. Some people swear you have to replace or rebuild the cylinders/calipers and replace the flex lines. Others just flush the system to replace all the DOT3/4 with DOT5. Seems like the people who replace everything have fewer problems but who knows.

Rick O.
02-01-2003, 10:42 PM
Few things to note: Get a gallon of denatured alcohol (ethanol) from Lowe's and run it through the system to remove the glycol and water. Then add the DOT5. Another is that you'll still have to renew the DOT5 every couple of years because water still gets in the system (it just pools). And finally, invest in a MityVac to bleed the system (wrap the bleeder screw threads in teflon tape first).

02-07-2003, 10:37 PM
Aloha Carmine,

I found this interesting discussion regarding brake fluids on the internet.


The idea of recycling the DOT 5 silicone brake fluid was something new to me. The article also has a detailed discussion of flushing your system and tips on bleeding the air out.

Safety Fast,

02-08-2003, 02:16 AM
Can anyone describe what DOT5 smells like?

02-11-2003, 06:20 PM
Aloha Scott,

Although my sense of smell is not that good, I can tell you that DOT 3 & 4 have a "banana oil" ester smell that reminds me of plastic model airplane glue from my youth. I can only say that DOT 5 doesn't smell like that. I don't have a good discription fpr DOT 5 other than it has more of a motor oil smell.

DOT 3, 4 & 5.1 are all glycol and glycol ether mixtures. There is also some brake fluids out there called "super DOT 4" and this is apparently similar to DOT 5.1. DOT 5 has a silicon chemistry. DOT 5 is also required to be purple, however the dye is only required to be stable enough to stay purple until it is poured from the original container. It is reported that over time (2 years or so) the DOT 5 in your car will lose the color and become clear or slightly amber.

DOT 5 is reported to be a good lubricant between medal and plastic and metal and rubber contact points which reduces wear. ABS brake systems apparently have some metal to metal contact points in them, DOT 5 is not a good lubricant in this situation, therefore not recommended in these systems.

DOT 5 is slightly compressable and might result in a spongy pedal feel, particularly in hard braking situations. This may make it unacceptable to some for racing applications. Additionally, since water and DOT 5 do not mix, water intrusion into the brake hydraulics (especially at caliper and wheel cylinders) will reduce the fluid boiling point from 500 deg F, to that of water.

The source for this information are various internet sites and Vintage Triumph Registry.

Safety Fast,

05-10-2003, 06:49 PM
Hello All,
Regarding the smell question, might I suggest a lot like the bimbo from baywatch...
Seriously, you folks seem to have it figured out pretty well, wish I'd read this when I started.

Silicone fluid is DOT5 and there is also DOT5.1 which is not the same, totally different stuff.

Previous post suggested flushing system with isoprproyl alcohol, excellent idea, even better if you find 91 or higher, percent alcohol, less water. Highest I have found is 98, at a paint supply house.

When filling system, biggest trick is pour slowly, and let sit to allow bubbles to evolve. Dot5 is a bit thicker than other fluids, and tends to hold bubbles in suspensiou for quite a while. Rushing things may migrate these throughout the system, thus the "spongy pedal syndrome"

Changed the fluids in several of my cars, loved the results. Worked well, lasted a long time! Never raced the stuff, but was driving in the mountains at the time...