Friday, March 24, 2023


Change your brake fluid for reliability and long system life

By Greg Illes
Absolutely everybody changes their engine oil and transmission fluid (or has it done). Some folks remember to change differential gear oil. But who ever thinks about changing brake fluid? We all should.

photo: wikimedia

Change brake fluid — Huh?

Why in the world would you want to do that? True, there are no grinding gears or slipping clutch plates in your brake system. But there are high temperatures, and there is a hidden secret in that fluid – it attracts water. The temperatures cause the system’s fluid to expand and contract slightly and this “breathes” tiny amounts of air in and out of the fluid reservoir. Over the course of time, each tiny puff of air contributes a small amount of water vapor which is absorbed into the fluid and circulated throughout your brake system.

Since water is as uncompressible as the brake fluid itself, you cannot tell from pedal pressure or system performance that the fluid has been compromised. But your system components can tell. That water starts to work its way into coatings and platings, starting the insidious corrosive process that can eventually ruin cylinders and calipers.

Rid the system of water

The only way to rid your system of the water is to completely flush the brake system with new fluid. This is very different from simply topping off the reservoir, and it’s not the same as emptying and refilling the reservoir. Flushing involves “bleeding” fluid from each and every wheel cylinder and pumping fluid through the system until fresh, new fluid flows out of the bleed valve.

The process is laborious and messy, and generally can require either two people and/or specialized equipment. Although I’m personally quite handy and do most of my own maintenance, this is one job that I leave to the pros. It will typically set me back about $100 or so.

Periodically flush the brake system fluid

A “brake job” is part of normal maintenance and will ordinarily consist of replacing pads or linings, turning drums or rotors and, of course, a thorough system inspection. By periodically flushing the brake system fluid, you can help to ensure that a routine brake job doesn’t end up involving costly replacement of corroded cylinders and calipers.

Since it’s time and not mileage that corrupts the fluid, the typical recommendation is for flushing every two years. Most of us have our rigs in the shop at least that often for one thing or another, so it’s easy to include the brake flush and not have to make it a special visit.

Greg Illes is a retired systems engineer who loves thinking up RV upgrades and modifications. When he’s not working on his motorhome, he’s traveling in it. You can follow his blog at



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Thomas D
2 years ago

Someone please explain to me how water migrates through the lines when a few centimetres is all it takes to move the calipers.Back and forth,back and forth. It really doesnt go from the master cylinder to the wheel caliper. Ive seen rusted brake lines. All from the outside. Salt,water,mud . You get the picture. In todays autos they are so reliable that things don’t break. Thats why they need to replace your brake fluid and change your oil at 3000 miles instead of manufacturers recommended distance of 5000/7500. They really need the work.

Ed M
2 years ago

I had to flush brake fluid after losing my brakes on a downgrade and having the moisture in the fluid turn to steam from the heat transfer from the pads through the piston and calipers. I was able to stop the motorhome with the parking brake but the experience was not pleasant.
I believe the mechanic changes from DOT 3 to DOT 4. He said it can take more heat.

Tom B
2 years ago

You can get a handheld vacuum pump from any major auto parts store to do this. It’s pretty easy. First, pump all the fluid out of the reservoir and replace with fresh fluid. Then go to each wheel, open the bleeder valve and hook up the pump. Pump until you see clear, fresh fluid. Be sure to check and top off the reservoir often so it doesn’t get empty, so you won’t suck air into the lines.

2 years ago

I think very few ever has the brake system bled. On newer vehicles with all the anti-lock, stabilization, anti-skid. It would probably be a very long, drawn out procedure and expensive at the dealer.
I have a Honda motorcycle and the suggested replacement of the brake fluid is 2 years, 24,000 miles. I do this religiously. When I flush the system, you can actually see the moisture floating in the fluid.
My dealer has never suggested the brake system flush.
I would imagine because of all the brake lines and valves in the newer vehicles, it would entail using a pressurized type bleeder.

2 years ago
Reply to  Bob

If you have seen moisture floating, then you have opened the cap in the rain. ALL brake systems utilize a diaphragm-type cap seal that will adjust the cap seal to the fluid level, that is, as the brake pads wear, the fluid level goes down and the seal has a diaphragm that adjusts accordingly. NO OUTSIDE AIR EVER enters unless you open the cap.
I agree that changing fluid is a good maintenance practice, but do not say water will get in from normal use, then you have a different issue.

Tommy Molnar
2 years ago

Funny . . . In my life, I have never done this, from back in the 60’s to now. And, no dealer or mechanic has ever suggested I do it. Now, if I was experiencing a spongy pedal, that’s a different story.

Larry Lee
2 years ago
Reply to  Tommy Molnar

You are not likely to experience a spongy pedal due to water in the system. Water is just as uncompressable as brake fluid. Spongy brakes indicate air in the system since air IS compressible. Water slowly causes corrosion throughout the system requiring virtually all components to be replaced at great expense. When flushing always use new, never before opened can(s) of brake fluid.

Tommy Molnar
2 years ago
Reply to  Larry Lee

Good point, Larry. You’re right about the air in the system. I wasn’t thinking straight – ha. Spongy pedal just requires a bleeding of the system.

Bernard McDermott
2 years ago
Reply to  Tommy Molnar

Just read gas the discusion on brakes and thought I could add info..I had a 36 ftl gas MH on a ford chasis. It had good brakes with a tag axle and a hydrobooster system,,But i was coming down a mountain in California and lost my brakes!! It was pucker time!! The RV was about 5 years old and had 20,000 miles on it brakes were fine. Until the moisture changes to steam and the petal went to the floor. parking brake lasted about 15 seconds. I was going about 45 when this started..Pumping would slow a bit , I downshifted and it slowed enough to get in 1 gear at app 25 MPH. I had a drop off on my right and a rock bluff on my left..I game across a pull off` and it had loose gravel , I got on the gravel and swerved back and forth while pumping like crazy and got STOPPED..Got towed in to a town about 50 miles and all it needed was new fluid and bleed the system. I religiously changed the fluid every 24 months after that experence..

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