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#4605311 - 12/16/17 11:28 AM Brake Fluid Testers
FlyPenFly Offline

Registered: 08/04/04
Posts: 236
Loc: San Diego
How accurate and effective are tools like this?

If you're just testing the fluid reservoir, is that accurate enough?

I have 2 vehicles, both with fluid factory 3.5 years old and they pass fine. Curious if that's accurate or what.
2014 Mercedes GLK350 3.5L DFI
2014 Mercedes E350 Coupe 3.5L DFI

#4605316 - 12/16/17 11:39 AM Re: Brake Fluid Testers [Re: FlyPenFly]
69GTX Offline

Registered: 09/23/15
Posts: 3680
Loc: Connecticut
I had my car's 13 year old brake fluid tested with strips at a Quick Lube place...and they told me it was still "good." In any event, I ended up changing it myself a few months later. I don't know how test strips can tell how much water or oxidation is present in the brake fluid.

2002 Lincoln Cont 4.6L DOHC/ 81K mi /PPlat 5w30/FUG XG2
1999 Camaro SS M6 /19K /Mobil 1 0w40 /Fram UG /GM MTL-ATF
1969 Ply GTX/RRs

#4605318 - 12/16/17 11:42 AM Re: Brake Fluid Testers [Re: FlyPenFly]
DBMaster Offline

Registered: 09/18/12
Posts: 5259
Loc: DFW
I'm curious about this, too. I have a similar tester that indicates my fluid is still good after 5.5 years and 83,000 miles. The fluid in the MC still looks straw colored. The tester measures moisture content in the fluid through conductivity.

#4605328 - 12/16/17 11:55 AM Re: Brake Fluid Testers [Re: FlyPenFly]
Superflan Offline

Registered: 04/28/15
Posts: 328
Loc: Reunion Island
Ok hereís what I think. Since thereís no cycling of fluid in the reservoir, the quality of fluid in the lines, abs actuators and calipers are undetermined.

Yet, on modern cars, pistons calipers are designed to not conduct heat generated when braking to the fluid, therefore claiming that no replacement is necessary during the normal life of the vehicle.

I had results confirming this on some cars, but on a car where I got brake vapor lock the test clearly showed a lowered boiling point of 120 degrees centigrade
Yaris 1.0 197mkm, Kangoo 1.5 DCi 113mkm 15W40 ACEA E7
Wife's Koleos 2.0 dCi DPF eco-map 154mkm 5W30 C3
Varadero 125 92mkm 10W40 JASO MA

#4605330 - 12/16/17 11:57 AM Re: Brake Fluid Testers [Re: FlyPenFly]
FlyPenFly Offline

Registered: 08/04/04
Posts: 236
Loc: San Diego
I guess there's an underlying assumptions here then: Moisture is the only reason to change brake fluid, is that true?

Flushing the brakes and testing the old fluid that comes out of the calipers also seems to be another data point to verify.
2014 Mercedes GLK350 3.5L DFI
2014 Mercedes E350 Coupe 3.5L DFI

#4605333 - 12/16/17 12:00 PM Re: Brake Fluid Testers [Re: FlyPenFly]
E150GT Offline

Registered: 03/11/12
Posts: 1015
Loc: San Antonio, TX
I have one of those cheap pens. I tested new brake fluid and it tested good. I took a small sample and added some water and it tested bad. I tested the reservoir in a 2003 tundra and it showed good. It looked terrible though.
1984 Mercedes-Benz 300SD - 100k
1995 F150 XL 4.9 reg cab 5 speed - 247k 5w30
2016 Mazda 6 manual 6speed - 46k 5w30

#4605342 - 12/16/17 12:24 PM Re: Brake Fluid Testers [Re: FlyPenFly]
BHopkins Offline

Registered: 11/03/13
Posts: 1071
Loc: Utah
I've also had the same question about the similar brake fluid tester that I own.

My plan is to mix up a test solution - 20 ml of fresh brake fluid with 1 ml of water for a 5% moisture solution, and see what results I get with the tester. Perhaps this might be something I can to this afternoon.
2013 Subaru Outback 2.5i - Royal Purple 0W-20 & NAPA Gold filter
2003 Oldsmobile Alero 3.4 - M1 5W-30 & NAPA Gold filter

#4605362 - 12/16/17 12:50 PM Re: Brake Fluid Testers [Re: FlyPenFly]
Marco620 Offline

Registered: 02/25/14
Posts: 1851
Loc: Deplorable in KS/ fly over
Dont have a tester but the Honda light will come on telling me brake fluid exchange is necessary.
2015 Civic Sp. Ed R18Z1 143k mi PUP0w20/Ceratec,HPS Intake,87e0 Borla Exhaust/Eibach&Tein Suspension
Go Nashville Predators!

#4605416 - 12/16/17 01:36 PM Re: Brake Fluid Testers [Re: FlyPenFly]
Ethan1 Offline

Registered: 12/29/14
Posts: 1620
Loc: 'murica
I'm surprised to hear the mixed reviews of the testers, because I never had reason to doubt the concept of testing resistance as a way to extrapolate H2O content. confused

#4605422 - 12/16/17 01:49 PM Re: Brake Fluid Testers [Re: FlyPenFly]
i_hate_autofraud Offline

Registered: 05/19/16
Posts: 673
Loc: Canada

It's more then H2O content, it's also oxidation and seal material wearing and clouding the brake fluid,
eventually this material precipitates and sticks to cylinder bores and cuts into seals when pistons are
pushed back to allow new pads to fit. Seen it!

Replace on a 2 or 3 year cycle is a safe bet compared to replacing an ABS unit when it malfunctions and stores
a pesky code! At that point you have 'regular' brakes!

For that reason, I don't use a brake fluid tester.

#4605428 - 12/16/17 01:59 PM Re: Brake Fluid Testers [Re: FlyPenFly]
Warstud Offline

Registered: 09/02/05
Posts: 5095
Loc: MIchigan
The test strips test for copper because : as your brake lines corrode.... more copper ends up in the fluid.

The test pens test for water.

#4605665 - 12/16/17 05:50 PM Re: Brake Fluid Testers [Re: FlyPenFly]
Hemispheres Offline

Registered: 01/14/17
Posts: 317
Loc: Virginia
Itís a good idea to change it a every few years due to crud more than moisture. Remove 1/3 of the fluid from resivior, fill back up and remove another 1/3 then vacuum bleed. Keeps the pistons and lines in good shape internally.
2009 Buick LaCrosse (3800 Series III) - 137,000 miles.
PZ Plat HM/Mobil 1 HM
Bosch/Purolator/Mobil filters

#4606308 - 12/17/17 02:27 PM Re: Brake Fluid Testers [Re: FlyPenFly]
barryh Offline

Registered: 01/09/16
Posts: 285
Loc: Cheshire, England
As far as water content is concerned I would say a reservoir test is representative. The hydroscopic nature of brake fluid will ensure any moisture diffuses throughout the whole system. As already mentioned there are other reasons to change brake fluid. I would want a test to show when the corrosion inhibitors had been used up but I suspect a reservoir test for that might not be representative of the whole system.

1979 BMW R45 (Triple QX 5W40)
2009 Mercedes C180K (Shell Helix Ultra 5W40)