brake fluid type?

[flyboytroy99]

My entire brake system is new. BIg brake package, master cylinder, etc. So should I use DOT 3 or DOT 5? Opinions anybody? I know you never mix the two.

[jordan]

I would wait for more of a consensus as this is very important. I myself would avoid DOT-5 as it is horrible, silicone based = perpetually spongy pedal.

DOT-3 is very hydroscopic (absorbs water) which lowers the boiling point of the brake fluid and is also very caustic on things such as paint.

I believe DOT-5.1 is Polyethylene glycol based which is not as compressible as the DOT-5 Silicone, has a high boiling point, and is not hydroscopic nor as caustic. All around 5.1 is the superior fluid for most applications; as a bonus it is compatible with DOT-3,4,5

Though if you have no problem with changing the fluid often DOT-4 may be the fluid of choice as it is hardly compressible being Glycol based, just as caustic on paint as DOT-3 though has a higher boiling point. If it is changed often then you won't have to worry about the low "wet boiling point"

[Jeepsbeme]

Originally Posted by jordan "DOT-3 is very hydroscopic (absorbs water)..."

Sorry to sound nit-picky, but the correct term is hygroscopic.

[heimue]

Originally Posted by jordan I would wait for more of a consensus as this is very important. I myself would avoid DOT-5 as it is horrible, silicone based = perpetually spongy pedal. DOT-3 is very hydroscopic (absorbs water) which lowers the boiling point of the brake fluid and is also very caustic on things such as paint. I believe DOT-5.1 is Polyethylene glycol based which is not as compressible as the DOT-5 Silicone, has a high boiling point, and is not hydroscopic nor as caustic. All around 5.1 is the superior fluid for most applications; as a bonus it is compatible with DOT-3,4,5 Though if you have no problem with changing the fluid often DOT-4 may be the fluid of choice as it is hardly compressible being Glycol based, just as caustic on paint as DOT-3 though has a higher boiling point. If it is changed often then you won't have to worry about the low "wet boiling point"

Jordon,

non of the fluids are compressible at all. Only gases are.

Dieter

[kwilford]

Originally Posted by heimue Jordon, non of the fluids are compressible at all. Only gases are. Dieter

True in theory, but brake fluids can become gasified which causes a spongy pedal. I believe that silicon fluids are more problematic for this as they tend to entrain air more readily than glycol fluids. Another consideration of either is that while glycol fluids absorb water (indeed the term is hygroscopic) which can cause fluid boiling and system corrosion, water tends to separate from silicon fluids into the low points in the system, which can cause even more severe corrosion.

JM2CW

[namba209]

Originally Posted by heimue Jordon, non of the fluids are compressible at all. Only gases are. Dieter

For all intents and purposes, that is a kinda sorta true statement, but not completly so. The Dean of American Aviation designers, Kelly Johnson, actually used a sealed cylinder with filled hydraulic fluid as a shock absorber on the F-104 main landing gear. The piston, acting on the fluid did in fact compress or recede one inch in the cylinder, with the dynamic forces of around 30,000 psi.

[wrench459]

I think that DOT 3 or 4 will work just fine in your application.
After a couple of years flush the old fluid out and add some fresh stuff.

[jordan]

Originally Posted by namba209 For all intents and purposes, that is a kinda sorta true statement, but not completly so. The Dean of American Aviation designers, Kelly Johnson, actually used a sealed cylinder with filled hydraulic fluid as a shock absorber on the F-104 main landing gear. The piston, acting on the fluid did in fact compress or recede one inch in the cylinder, with the dynamic forces of around 30,000 psi.

I know there is a difference between laws and or theory and actual test results, depending on the volume of fluid in the cylinder of the F-104 landing gear I would be more inclined to believe that it was a cylinder yield than a compression of fluid. Unless the fluid reached a supercritical point, which at 30,000psi it is quite possible (volume dependant)

Also when you get into polymers some interesting things can happen. When in solution polymers can be compressible especially as a supercritical fluid because the silicone is a non crystalline flexible lattice, specific volume may vary.

[BMWonly]

I know the classic BMW community swears by DOT 4. It has a higher boiling point than DOT 3. DOT 5 is ok in newer cars, but could result in rusty calipers in a car that is not driven on a regular basis.

"Direct from Germany to your BMW. Ate Super Racing brake fluid from Germany features a superior formulation that provides the more reliable performance and longer wear your BMW's braking system must have, especially if you drive hard. Blue color to make fluid changes visibly easier. 1 liter. Exceeds Dot 4 standards. Boiling Point 536 degrees. "

ATE BLUE Ate Super Blue Racing Brake Fluid DOT 4 - 1 liter.
$13.95


Dieter

[DennisGardiner]

I will recommend DOT 4. My personal preference is Castrol LMA
Castrol USA - Brake Fluid
Great stuff, I have been using it for years.

I have boiled DOT 3 before with my big brakes when driving "HARD".

Have yet to boil the Catrol LMA,... does not mean you can't (because you can)
I just never have as of yet.

[RallyBob]

Castrol LMA is inexpensive and usually very available, a big plus. I admittedly use it most of the time. However, for those times when it's simply not enough, I prefer ATE Blue, AP 600 or Motul. I've boiled the LMA many times in performance driving.

Regardless of the fluid brand, change it every two years at least. Change it every year if the car sits most of the time...NOT using the brakes is the worst thing for them, if you will never get them up to temp any moisture in the system settles out rather than evaporates. If you race, change it at least every other event.

[hrcollinsjr]

Originally Posted by BMWonly I know the classic BMW community swears by DOT 4. It has a higher boiling point than DOT 3. DOT 5 is ok in newer cars, but could result in rusty calipers in a car that is not driven on a regular basis. Dieter

If it doesn't readily absorb water how is it going to contribute to rusty calipers? My understanding was that DOT 5 was a good choice for classics and seldom driven cars for preventing rust.

I prefer Castrol LMA, but it is a little more difficult for me to find in a small town. IIRC I stock up on it when I go to Nashville.

Harold

[Jeepsbeme]

My first restoration (a '39 Chevy pick-up) would always exibit wheel cylinder problems after sitting all winter with DOT-3. In 1985 I switched over to silicone and never had a problem since...and even the current owner has yet to experience any problems.

I then ran silicone in my next project - a '76 jeep used both on the road and HARD off-road. I never had any problems in the 15+ years I drove it.

My Pacer also has had silicone fluid for 10 years without a problem.

I've read about the supposedly spongier pedal feel of silicone fluid, but if it truly is softer - I can't pick up on the difference. Nor has anyone else that's driven any of my toys.

There's no question that if you do get moisture in a silicone fluid, that it tends to settle in one spot instead of getting dispersed throught the system. But silicone also doesn't absorb moisture from the atmosphere. So I'm assuming the only likely source of moisture contact is if water gets into the master cylinder through the vent in the cap. Even at that, most master cylinders have a rubber bellows between the fluid and the atmosphere so there's no direct contact.

An engineer from Dow Chemical once told me conventional brake fluid actually slowly absorbs moisture right through the rubber of flex lines! But being non-hygroscopic, this isn't an issue with silicone fluid.

My personal opinion is silicone is hard to beat for most project vehicles.

[jordan]

Originally Posted by hrcollinsjr If it doesn't readily absorb water how is it going to contribute to rusty calipers? My understanding was that DOT 5 was a good choice for classics and seldom driven cars for preventing rust. I prefer Castrol LMA, but it is a little more difficult for me to find in a small town. IIRC I stock up on it when I go to Nashville. Harold

As Brian said in the last response the reason for the rusting with non-hygroscopic fluids is most likely because the fluid will not allow the water into solution so it will stay heterogeneous and in contact with metal. And not being in solution you have water ready to vaporize at 212 degrees F. which for brakes can happen very easily and then you have a pocket of compressible vapor that you can never pump past because it expands as you let off of the brake and another pump to get more pedal won't get you anywhere until it cools. There are benefits to all types of brake fluids though minimal between 3 and 4. If the maintenance effort is made I personally see no reason to use non glycol based fluid.