Brake Fluid Change and moisture level

[Americanrascal]

2016 F53 26000lb chassis. I regularly check the fluid and moisture levels in the brake fluid. I use a tester to measure the moisture and it is consistently in the safe zone at less than 1% moisture. I changed the fluid about 3.5 years ago.

Ford manual calls for a 2 years change out period on brake fluid . Is there any other reason than higher moisture levels in brake fluid that would require change at that frequency?

I am not aware of any other reason to change it other than excessive moisture, but I may be missing something. Please let me know if it is essential to change if brake fluid moisture content is well within safe limits. Thanks-Joe

[Ray,IN]

Reason #2, exposure to high heat from prolonged braking degrades brake fluid.

[TeJay]

RAY,IN is correct. Heat will cause brake fluid to deteriorate. In addition every 1 to 2 years may still allow brake fluid to absorb water. Much of that depends on the quality of your braking system. How can water absorb into the system? Rubber seals, Master cylinder cap seal, caliper seals and others. They will deteriorate over time allowing the fluid to absorb water.

A book put out by the DOT stated that most vehicles will have 1 +% of water within the brake system when you buy it. Flushing the system every 2-3 years will greatly assist the fluid staying nice and clean.

[AITG]

One other benefit to changing brake fluid regularly is keeping the bleeder screws from seizing up. Depending on where live that may not be a problem.

[onecb]

Well I live in a very dry area Salt Lake, I change the fluid at the two year mark regularly. When changing I can definetly see the new start coming out the old is a rust color real noticable. So it is got some water in it to show that much rust. This is just my own observation Alvin

[Hikerdogs]

Brake fluid does not circulate through the system. It remains in a nearly stationary position advancing toward the caliper as the piston is extended and returning to near its original position when the brake pedal is released.

Moisture (water) is heavier than the brake fluid and eventually seeks the lowest point in the system. That point is usually at or near the caliper. When the brake pedal is depressed, the water can enter the caliper. Since it has a boiling point of 212*F it will boil and vaporize long before the brake fluid.

The heat generated by braking can cause the water to vaporize. It then becomes a compressible gas and will not force the caliper piston to extend nearly as well as the liquid brake fluid. You will experience "brake fade" when the water content exceeds 3%. Higher amounts of water can result in brake failure.

Keep in mind that since the water gravitates to and is trapped at the lowest point(s) in the system testing fluid in the reservoir will not necessarily be accurate as to the content throughout the system.

Water trapped in the steel brake lines can also cause them to rust from the inside out. You will have no idea there is a problem until the line ruptures and brake fluid is lost. The fluid loss will result in diminished braking ability.

That's why today's vehicles have multiple chamber master cylinders. If a line or lines from one half are damaged the remaining half will provide fluid to the remaining intact circuit.

[Americanrascal]

OP update: Will be changing fluid in a week when 5.1 arrives. Thanks for a;ll the input.

[Dodge Guy]

Every 2-3 years depending on use. No longer than 3 years.

[jpstakun]

Water does not stay in suspension with brake fluid. It separates and precipitates to the bottom level, ie: at the bottom of the master cylinder. This water sits there and causes the cast iron piston to corrode leading to fluid bypassing the O rings causing brake failure, especially when using the brakes heavily going down a mountain. The water then turns to vapor which is compressible, that's why you don't get any braking action. In extreme conditions, you may have to replace the master cylinder and the calipers. Flushing the brake fluid every 2-3 years is a wise thing to do. Be sure your mechanic flushes all the lines and the calipers as well.

[Americanrascal]

OP here. Great info from everyone. I finished the job this afternoon . Drained the master cylinder (sucked it out). Loaded (and kept loading it ) with DOT 5.1. Then went around from right rear to left rear to front right to front left cylinders pulling and pumping out the lines, reloading with fresh fluid after each wheel.

Out of curiosity I tested the fluid from each wheel and frankly they read that they were loaded with moisture. Not in the danger level but certainly enough to show significant moisture. You may recall my original test on the resevoir in the master cylinder showed virtually no moisture. However each wheel draining reflected extreme moisture.

Pedal seems nice and firm so hopefully no air in the system

So glad I did the job. Learned a lot from you all and the experience.

I could not find the actual capacity of the system in any manuals, but it took about 1.5 qts to do the job refilling the resevoir anda after each wheel.

Thanks Joe