Dot 3 or Dot 5 Brake fluid?

[derby]

Ok, I would like to get a general opinion on the two fluids. I have hrard posative things on the Dot 5,and a couple negative.Is it really worth the extra expense?

 

[gimpyrobb]

Well I think that it (dot5) is supposed to not hold water, therefore less rust in the brake system. Take it or leave it.

 

[papercu]

Fluid

If you drive your truck regular DOT 5 is not needed, the heat removes any water that might be in the system. When was the last time you changed the brake fluid in your car?
BTW I think you ARE suppose to change it ever so often but it's rarely done. Wayne

 

[acetomatoco]

RE: Fluid

don't tell anyone, but I mix em.. ACE

 

[JDToumanian]

I don't know if it's just a coincidence or not, but my truck had DOT 5... I switched to DOT 3 and now everything leaks. Started leaking very soon after the change, noticed it a few days later. However, after I rebuild 6 wheel cylinders, an air pack and a master cylinder, I will probably still use DOT 3.

Jon

 

[sermis]

I flushed my system and went back with DOT 5. Lot of cost. From all I have read and talked to people is that DOT 5 has good water properties and will help with water / rust in the system. A lot of military vehicles had been running DOT 3 and were converted to DOT 5 to keep the lines from freezing in extrem cold. If I were to do it again I would go with DOT 3 just for cost. I can't see how either would cause any failure with rubber parts in the system.

 

[HeadWizard]

Speaking from daily experience, we see many fewer hydraulic brake problems with customers who flush their brake fluid regularly. Brake fluid is hygroscopic (attracts moisture from the atmosphere). We recommend a full fluid flush any time the tester senses more than 3% moisture content or whenever a brake job is done.

Provided the brake fluid is changed every 2 years or so, it probably does not matter what kind you use. We use BG Products DOT 4 fluid for everything we do at the shop. It has a higher boiling point and attracts less moisture than DOT 3. It is still a Glycol based fluid, rather than the normal silicone base of DOT 5. There are some DOT 5's around that are still organic and if you decide to use a DOT 5 use one of the non-silicone ones.

Silicone brake fluids generally do not absorb moisture, but rather displace it. What this means to your brake system is that the moisture will "puddle" in the brake system causing water to be in direct contact with the metal (lines, master cylinders, airpack units, wheel cylinders). These puddles will rust the inside of your brake hydraulic system.

I just disassembled my airpack unit yesterday after a malfunction. I found that the seals were still very pliable and that for the most part the unit was OK - with the exception of the small end of the cylinder being rusted and an apparent rust build up at one of the seals - from water in the system. I honed the bores to a nice finish and used 600 grit wet/dry where the hone would not go. It reassembled very well and I expect that the problem will be gone. I contacted the previous owner and he said he'd been using silicone brake fluid per the dash sticker. Cause of the problem found. I just hope the system will bleed out to a firm pedal.

Bottom line: Flush your brake fluid at least every 2 years and do not use silicone fluid. Your future brake hydraulic problems will disappear.

One mans opinion.

 

[CGarbee]

I thought that I would do a "cut and paste" from two posts on the www.g741.org forum since folks keep asking this question over there...

One from one of the other guys who tends to write thoughtful, well researched posts:
First of all, it's important to understand the chemical nature of brake fluid. DOT 3 brake fluids are mixtures of glycols and glycol ethers. DOT 4 contains borate esters in addition to what is contained in DOT 3. These brake fluids are somewhat similar to automotive anti-freeze (ethylene glycol) and are not, a petroleum fluid. DOT 5 is silicone chemistry.
Brake system materials must be compatible with the system fluid. Compatibility is determined by chemistry, and no amount of advertising, wishful thinking or rationalizing can change the science of chemical compatibility. Both DOT 3-4 and DOT 5 fluids are compatible with most brake system materials except in the case some silicone rubber external components such as caliper piston boots, which are attacked by silicon fluids and greases.
The big bugaboo with DOT 3-4 fluids always cited by silicone fluid advocates is water absorption. DOT 3-4 glycol based fluids, just like ethylene glycol antifreezes, are readily miscible with water. Long term brake system water content tends to reach a maximum of about 3%, which is readily handled by the corrosion inhibitors in the brake fluid formulation. Since the inhibitors are gradually depleted as they do their job, glycol brake fluid, just like anti-freeze, needs to be changed periodically. Follow BMW's recommendations. DOT 5 fluids, not being water miscible, must rely on the silicone (with some corrosion inhibitors) as a barrier film to control corrosion. Water is not absorbed by silicone as in the case of DOT 3-4 fluids, and will remain as a separate globule sinking to the lowest point in the brake system, since it is more dense.
Fluid boiling point DOT 4 glycol based fluid has a higher boiling point (446F) than DOT 3 (401F), and both fluids will exhibit a reduced boiling point as water content increases. DOT 5 in its pure state offers a higher boiling point (500F) however if water got into the system, and a big globule found its way into a caliper, the water would start to boil at 212F causing a vapor lock condition . By contrast, DOT 3 fluid with 3% water content would still exhibit a boiling point of 300F. Silicone fluids also exhibit a 3 times greater propensity to dissolve air and other gasses which can lead to a "spongy pedal" and reduced braking at high altitudes.
DOT 3 and DOT 4 fluids are mutually compatible, the major disadvantage of such a mix being a lowered boiling point. In an emergency, it'll do. Silicone fluid will not mix, but will float on top. From a lubricity standpoint, neither fluids are outstanding, though silicones will exhibit a more stable viscosity index in extreme temperatures, which is why the US Army likes silicone fluids. Since few of us ride at temperatures very much below freezing, let alone at 40 below zero, silicone's low temperature advantage won't be apparent. Neither fluids will reduce stopping distances.
With the advent of ABS systems, the limitations of existing brake fluids have been recognized and the brake fluid manufacturers have been working on formulations with enhanced properties. However, the chosen direction has not been silicone. The only major user of silicone is the US Army. It has recently asked the SAE about a procedure for converting from silicon back to DOT 3-4. If they ever decide to switch, silicone brake fluid will go the way of leaded gas.
Brake system contamination
The single most common brake system failure caused by a contaminant is swelling of the rubber components (piston seals etc.) due to the introduction of petroleum based products (motor oil, power steering fluid, mineral oil etc.) A small amount is enough to do major damage. Flushing with mineral spirits is enough to cause a complete system failure in a short time. I suspect this is what has happened when some BMW owners changed to DOT 5 (and then assumed that silicone caused the problem). Flushing with alcohol also causes problems. BMW brake systems should be flushed only with DOT 3 or 4.
If silicone is introduced into an older brake system, the silicone will latch unto the sludge generated by gradual component deterioration and create a gelatin like goop which will attract more crud and eventually plug up metering orifices or cause pistons to stick. If you have already changed to DOT 5, don't compound your initial mistake and change back. Silicone is very tenacious stuff and you will never get it all out of your system. Just change the fluid regularly. For those who race using silicone fluid, I recommend that you crack the bleed screws before each racing session to insure that there is no water in the calipers.
Since DOT 4 fluids were developed, it was recognized that borate ester based fluids offered the potential for boiling points beyond the 446F requirement, thus came the Super DOT 4 fluids - some covered by the DOT 5.1 designation - which exhibit a minimum dry boiling point of 500F (same as silicone, but different chemistry).
Additionally, a new fluid type based on silicon ester chemistry (not the same as silicon) has been developed that exhibits a minimum dry boiling point of 590F. It is miscible with DOT 3-4 fluids but has yet to see commercial usage.
By Steve Wall

One of mine:
The problem with using DOT 5 in these vehicles (M37 that came with DOT3 originally) is that you have to be careful bleeding the brakes in order to ensure that you get all the air out (the viscositiy of DOT5 is such that minute air bubbles tend to get suspended in the fluid and it takes a while for them to work out, handle the stuff gently...), and the water that does get into the system tends to pudle in the low spots and cause rusting of lines (so you have to flush the system just as often as with water absorbing DOT3/4). DOT5 does not remove paint like DOT3/4 though, so it is a good choice if you tend to spill fluid on the bodywork when topping off the MC.

The DOD has converted the existing fleet of vehicles over to DOT5 in order to extend the temperature range that the vehicles may be operated in without having to switch fluids. Studies are underway to determine the feasabilty of converting the fleet back to DOT4...

So, I'd stay with DOT5 if it came to me with it in the brake system, and DOT3/4 if that was installed.

Good luck.

 

[derby]

Is DOT5 always purple? the fluid that was in my truck was a greenish/yellow almost flourescent color.thought maybe the Military's was differant. anyways thanks for the insight.

 

[CGarbee]

My experience:
DOT5 isn't always purple... Even the purple versions (most all is) tends to loose its color when in the system... It all turns sorta straw colored over time.

 

[sermis]

There is allways the tast test. Bitter DOT 3 or 4. Not bitter Dot 5.

 

[littlebob]

I'm going with the Dot5, because I've heard it's better for collector vehicles that don't get driven regulary. I know from experience that a vehilcle that sits will accumulate mositure if not driven long enough on a regular basis to boil out the water. I would use the much cheaper 3 or 4
if it was for a dailly driver and change on a regular basis.
littlebob

 

[Floridianson]

There is allways the tast test. Bitter DOT 3 or 4. Not bitter Dot 5.

Don't tell me you do that. I have done antifreeze that way but could be the reason I see Unicorns when no one else is around.

 

[bigmike]

Don't tell me you do that. I have done antifreeze that way but could be the reason I see Unicorns when no one else is around.

Lucky you, I keep having conversations with the carebears.

 

[Floridianson]

Lucky you, I keep having conversations with the carebears.

Don't see a problem with that just don't take any financial advice from them.

 

[dmetalmiki]

14 trucks in our 'fleet' all any any of them drained on acquisition, and dot4 placed in all of them.
All good so far. (And that's after 12-14 years!)
They are all 'drained through' every two or three years.