I understand that there are many types of brake fluid...What's the best choice for one of our cars...a 39 P-8, Thanks,Dennis
You'll get a lot of different opinions but I use DOT-5 Silicone brake fluid. It won't hurt your paint and it does not attract or hold moisture. It is widely used in older cars that do not get driven everyday. I made a pressure bleeder from a garden hand pump sprayer. The hardest part is finding a spare reservoir cap to drill and tap for a hose fitting.
Jim
I too am using DOT-5 (silicone) brake fluid and am happy with it. It does entrain air easily if you are not careful when bleeding. And there is some antidotal evidence that hydraulic brake light switches don't last as long though I haven't had any issues in 10 years.
However there appears to be a direct correlation between having issues with DOT-5 and not having a brand new brake system. That is nearly all the problems I have heard about were on cars that previously had DOT-3 fluid and did not have all new or rebuilt components. When I put my brakes together everything including all tubing and hoses were either new or fresh rebuilds. This seems to be important. So if you already have brake fluid in your system and are not going to rebuild/replace everything I'd recommend staying with what is in the system now.
Can I add a DOT-5 (Silicone) fluid to (whatever) brake fluid is existing in there-?...Dn't know what type is in there. Thanks,Dennis
NO!! if you convert to Dot-5 the system must be flushed completely of whatever is in there now
My outlook is brake fluid is brake fluid. Just stick to whatever non-silicone (dot4 and below) is on your parts store shelf.
I replaced my dot-3 with dot-5. and then replaced every piece of rubber in the system because it melted away. But no problems since then. jim
Dot 5 Silicone will not absorb water, will not rust, but You have to really clean Your system before and use Silicone- proof rubbers. Worth wile when redoing the whole system.
I got the hint 15 years ago from one of Your tank officers in Schweinfurt, Germany, they used it in the long- time- stored equipment to avoid any need of servicing. Later I talked to his brother, US- Coast Guard, using the same stuff in their steering hydraulics. No heat and cold problems for them.
Greetings! Go
Early in my carrier I twisted wrenches for the government and have converted several vehicles over to Dot-5 with only a couple of soft pedal problems.? We warranted the master cylinder with no other problems.? The biggest mistake made is flushing the system out especially with alcohol you Will be replacing rubber components Guaranteed!!? You can just bleed the Dot 5 through, and replace nothing unless it needs replacing anyway.? Now would be a great time to inspect every thing & replace items in question.? Make sure you do a though job of bleeding as it tends to turn the petroleum based fluid into mud down the road if you did not get it all out. A power bleeder is the best way if you have access to one and make sure the fluid coming out of the bleeder is clean, that way you should not have any residual old fluid.? But if you experience that, I just clean the mud out with a clean cloth and rebleed till you get the blue or red fluid flowing clean!.? The only advantage you would gain is corrosion proofing and that ant all bad at all!!? The petroleum based fluid can absorb moisture causing lines to corrode from within over time, the silicone will not absorb moisture and that's a good thing!? The other advantage to Dot 5 is brake heat dissipation from repeated hard applications that is probably not the way you are driving your car.? Now if you are experiencing brake fade do to heat then you will notice a difference.? The question I guess is it worth the extra cost converting over?? Or if it ain't broke don't fix it?
Here is a article that is a must read that should help.
Battle of the DOTs
DOT 3-4 Verses DOT 5. Which brake fluid should I use?
As a former materials engineering supervisor at a major automotive brake system supplier, I feel both qualified and obligated to inject some material science facts into the murky debate about DOT 5 verses DOT 3-4 brake fluids. The important technical issues governing the use of a particular specification brake fluid are as follows:
1. Fluid compatibility with the brake system rubber, plastic and metal components.
2. Water absorption and corrosion.
3. Fluid boiling point and other physical characteristics.
4. Brake system contamination and sludging.
Additionally, some technical comments will be made about the new brake fluid formulations appearing on the scene.
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, as Dr. Curve implies, a petroleum fluid. DOT 5 is silicone chemistry.
Fluid Compatibility
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.
Water absorption and corrosion
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 [possible brake failure -ed.]. 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.
New developments
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.
This is my first post as a new member so I wish to apologize for being so lengthy.
Herb
No apology need - lots of good information - thanks!
Mark