Brake Fluid?

[riggz]

I am swapping out the rotors and pads on my 09 Suburban today and thought I would go ahead and flush the brake fluid while I am at it.

I've read to use DOT3 or 4 but noticed the GM brake fluid is glycol based and most of the brake fluids at the store were DOT3/4 synthetic.

I know you shouldn't mix the two but if I flush the whole system with synthetic is that going to cause any issues? Or should I just try to find glycol fluid?

 

[ulakovic22]

A full flush should be sufficient. A lot of guys will alternate colored fluids so they can see when the old stuff is out and the new stuff in. The key will be opening the abs valves to flush the whole system and not just the lines.

 

[OR VietVet]

Dot 3/4 is not synthetic. Dot 5 is. Unless they have changed things from years ago.

 

[AZSCTACO]

Dot 3/4 is not synthetic. Dot 5 is. Unless they have changed things from years ago.

Bottle says Synthetic.....

 

[OR VietVet]

Sorry, but did not used to be that way. I think though that the "Dot" number relates to the capability to absorb heat.

 

[TheAutumnWind]

Sorry, but did not used to be that way. I think though that the "Dot" number relates to the capability to absorb heat.

Minimal boiling points for these specifications are as follows (wet boiling point defined as 3.7% water by volume):

Boiling point ranges [2]
Dry boiling point Wet boiling point
DOT 3 205 °C (401 °F) 140 °C (284 °F)
DOT 4 230 °C (446 °F) 155 °C (311 °F)
DOT 5 260 °C (500 °F) 180 °C (356 °F)
DOT 5.1 260 °C (500 °F) 180 °C (356 °F)

 

[Freedom Motorsports]

DOT 4 is better than DOT 3 due to higher wet boiling point. Do NOT use DOT 5, it is for race specific applications ONLY and will cause damage. If you are looking for the best overall fluid and are buying locally, synthetic claims aside, check out the specs for wet boiling point of those you have available to you and pick the one with the highest wet boiling point. It will last the longest after being saturated with moisture. Also keep in mind that brake fluid, once opened, has little to no shelf life due to it's formulation causing it to absorb moisture. For what it's worth, Amsoil Series 500 DOT 3 has one of the higher wet boiling points in the industry and is synthetic. Only 3.7 percent water contamination is often sufficient to reduce a DOT 3 fluid’s boiling point below the DOT minimum. You can check out the specifics below.

http://www.amsoil.com/shop/by-produ...e-dot-3-brake-fluid/?code=BF3SN-EA&zo=1678239

 

[TheAutumnWind]

Dot 3/4 is not synthetic. Dot 5 is. Unless they have changed things from years ago.

Your thinking of glycol vs silicone based probably IIRC there is such thing as non synthetic dot3, but there is synthetic versions as well.

wiki info

DOT 4, like DOT 3 and DOT 5.1, is a polyglycol ether-based fluid (contrasted with DOT 5 which is silicone-based). Fluids such as DOT 3, 4, and 5.1 are hygroscopic and will absorb water from the atmosphere. This degrades the fluid's performance, and if allowed to accumulate over a period of time, can drastically reduce its boiling point. In a passenger car this is usually not much of an issue as the brakes are generally not used so hard, but can be of serious concerns in racecars or motorcycles due to the higher heat generated during their much more aggressive braking.

 

[OR VietVet]

Before the questions start here is an explanation. Yes water does get in to the brake fluid because the system has to vent. As you use your brakes heat is built up and the fluid will absorb that heat that is not dissipated throughout the rest of the brake system. When the brake fluid absorbs heat it will expand and therefore the brake system has to be vented or as the fluid expands the pressure will build in the system and brakes can apply like dragging brakes caused by laying your foot on the brake pedal while driving.
The brake fluid then can absorb the moisture in the air, it is hygroscopic and as more and more moisture is absorbed the boiling point lowers and cause damage internally over time and can cause brake fade from the break down of the fluid. Clear as mud?

 

[Freedom Motorsports]

I have to respectfully disagree with the operational explanation there. Brake fluid absorbs condensation in the braking system that is a direct result of heat cycling. When you apply the brakes, they get warm/hot, but while driving, in most situations, they cool quickly for the most part and that cycling causes condensation in the system. Brake fluid is designed to absorb moisture/condensation so that pockets of moisture do not form in the calipers or other parts of the system. There isn't any venting in the brake system that is exposed to external moisture, if there were, the brake fluid would wear out/ degrade so fast that you'd be changing it every other month. Brake fluid can absorb moisture from the air so quickly, it is advisable that when you are bleeding your brake system, keep the reservoir cap in place unless you are adding fluid. The only openings in the system where the fluid is concerned are at the reservoir and bleeder valves on the calipers.

---------- Post added at 12:34 PM ---------- Previous post was at 12:31 PM ----------

Additionally, the pressure in the brake system should be constant when running, the only time the pressure should lower is when there is a problem. Whether it be with bad fluid that has broken down or there is a leak in the system causing it to lose fluid or another issue pertaining to either a vacuum leak for booster style systems or worn out power steering fluid or hydroboost units one vehicles equipped with hydroboost systems.