Is it advisable to flush the brake fluid in 35 C (95 F) weather with 80% humidity?

[matthewr87]

I have a track day at Watkins Glen coming up and I'd like to flush my brake fluid. However the weather here has been terribly hot and humid. I feel like if I try to do the flush with 80% humidity I'll just end up introducing more water into the system than I have now (<2%). Or will it not matter?

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[pilotgore]

I wouldn’t sweat it. Just make sure you’re using new unopened bottles and limit the amount of time the cap is off of the reservoir, brake fluid bottles, and pressure bleeder bottle if you’re pressure bleeding. (These are all best practices for bleeding in general.)

Have a good time at the track!

 

[WildHorse]

I'll just end up introducing more water into the system than I have now (<2%).

How'd you determine that ?

 

[jmn444]

How'd you determine that ?

i googled... they actually have reasonably priced testers! not sure exactly what our % threshold should be for track use though, but kinda neat.

wonder if the % is generally stable thruout the system or if the reservoir would test higher/lower than the rest or whatever too....

 

[Tomster]

Flush with new fluids from a sealed bottle. The amount of air in the resevoir is nothing compared to as if you were using a bottle that was opened a long time ago.

Negligible if you ask me.

 

[matthewr87]

Okay sounds good. Thanks guys.

This is what I used to check the water level:

https://www.amazon.com/PTE-Tester-C...ords=dot+4+fluid+tester&qid=1630174291&sr=8-3

Whether water content is uniform throughout is an interesting question. I do not think there is any real flow to the brake fluid once it is in the lines and calipers. So water content would have to diffuse passively through the fluid. I imagine it would reach an equilibrium at some point. Also depends where the intrusion points are.

 

[oldbmwfan]

Flush with new fluids from a sealed bottle. The amount of air in the resevoir is nothing compared to as if you were using a bottle that was opened a long time ago.

Negligible if you ask me.

For grins, I recently tested several bottles of Motul RBF600 that had been opened and partially used in 2016, 2017, 2018, and 2021. All had been opened, used to flush and bleed a system, and re-capped and stored in a drawer in my tool box since use. They all tested 0% water.

I did brake fluid flushes on three of my cars in the same week because I took all three to a private track rental (Grattan) in July. A couple friends came in and drove my M5 and Fiesta ST, and I drove the 993. After testing the old fluid, I used up all the old bottles while doing the flushes. None of the cars had any problems on track. Close the lid properly and tightly after use, and it apparently doesn't go off for a long time. YMMV.

 

[pilotgore]

For grins, I recently tested several bottles of Motul RBF600 that had been opened and partially used in 2016, 2017, 2018, and 2021. All had been opened, used to flush and bleed a system, and re-capped and stored in a drawer in my tool box since use. They all tested 0% water.

I did brake fluid flushes on three of my cars in the same week because I took all three to a private track rental (Grattan) in July. A couple friends came in and drove my M5 and Fiesta ST, and I drove the 993. After testing the old fluid, I used up all the old bottles while doing the flushes. None of the cars had any problems on track. Close the lid properly and tightly after use, and it apparently doesn't go off for a long time. YMMV.

Very interesting. Thanks for doing what most of us (including me) were too lazy to do

 

[oldbmwfan]

Very interesting. Thanks for doing what most of us (including me) were too lazy to do

I had mostly been keeping it around for use when rebuilding calipers, which is something I had to do quite frequently when tracking cars with single-piston floating caliper designs, but it seemed wasteful so I thought I'd check it. I was quite surprised by the results. My older containers of Ate Typ200 and such I am still keeping as clean, old fluid for rebuilds, but only because that fluid has a lower total performance than the Motul, no longer because of concern about water contamination.

 

[pilotgore]

I had mostly been keeping it around for use when rebuilding calipers, which is something I had to do quite frequently when tracking cars with single-piston floating caliper designs, but it seemed wasteful so I thought I'd check it. I was quite surprised by the results. My older containers of Ate Typ200 and such I am still keeping as clean, old fluid for rebuilds, but only because that fluid has a lower total performance than the Motul, no longer because of concern about water contamination.

I wonder if it’s worth leaving the cap off of a bottle for a week and testing again, as a control.

 

[NoXiDe]

For grins, I recently tested several bottles of Motul RBF600 that had been opened and partially used in 2016, 2017, 2018, and 2021. All had been opened, used to flush and bleed a system, and re-capped and stored in a drawer in my tool box since use. They all tested 0% water.

I did brake fluid flushes on three of my cars in the same week because I took all three to a private track rental (Grattan) in July. A couple friends came in and drove my M5 and Fiesta ST, and I drove the 993. After testing the old fluid, I used up all the old bottles while doing the flushes. None of the cars had any problems on track. Close the lid properly and tightly after use, and it apparently doesn't go off for a long time. YMMV.

I've done the same as well but I will admit I do throw them in a ziploc bag to maintain a "seal." No issues either.

 

[oldbmwfan]

I wonder if it’s worth leaving the cap off of a bottle for a week and testing again, as a control.

I will do that. It's been humid as a prostitute's panties here the last couple weeks, so should be a good test. I hope to get by the shop tomorrow, so I'll crack open a bottle and take a "before" fluid test.

 

[oldbmwfan]

I will do that. It's been humid as a prostitute's panties here the last couple weeks, so should be a good test. I hope to get by the shop tomorrow, so I'll crack open a bottle and take a "before" fluid test.

Okay, I got around to doing this. I used a bottle of brake fluid that had been open in 2016, then closed tightly and stored in a drawer in my climate controlled shop (heated, no AC). As mentioned before, I keep this fluid around for things like caliper rebuilds.

So, the results:
Out of the capped bottle, 7 years old, I poured some fluid into another bottle cap to sit out. Right after pouring it, photo taken 8/31. 0% water, so the fluid is good

So I left that capful of brake fluid sitting out about 10 days. Here it is on 9/9:

Clearly has absorbed a ton of water from the air.

To be sure, I poured a fresh capful from the same bottle as the first, which I had re-capped normally:

So, my conclusion: all this talk about "trash any unused brake fluid after the bottle is open" is total crap. Re-seal the bottle tightly store it indoors, and you're fine for at least 7 years. Earlier this summer I used similar fluid from 2018 and 2019 on track, and it was fine.

I actually feel much better for having done this little test.

 

[pilotgore]

Okay, I got around to doing this. I used a bottle of brake fluid that had been open in 2016, then closed tightly and stored in a drawer in my climate controlled shop (heated, no AC). As mentioned before, I keep this fluid around for things like caliper rebuilds.

So, the results:
Out of the capped bottle, 7 years old, I poured some fluid into another bottle cap to sit out. Right after pouring it, photo taken 8/31. 0% water, so the fluid is good

So I left that capful of brake fluid sitting out about 10 days. Here it is on 9/9:

Clearly has absorbed a ton of water from the air.

To be sure, I poured a fresh capful from the same bottle as the first, which I had re-capped normally:

So, my conclusion: all this talk about "trash any unused brake fluid after the bottle is open" is total crap. Re-seal the bottle tightly store it indoors, and you're fine for at least 7 years. Earlier this summer I used similar fluid from 2018 and 2019 on track, and it was fine.

I actually feel much better for having done this little test.

That’s incredibly helpful! Thanks for taking the time to run the tests and post the results!

 

[matthewr87]

Okay, I got around to doing this. I used a bottle of brake fluid that had been open in 2016, then closed tightly and stored in a drawer in my climate controlled shop (heated, no AC). As mentioned before, I keep this fluid around for things like caliper rebuilds.

So, the results:
Out of the capped bottle, 7 years old, I poured some fluid into another bottle cap to sit out. Right after pouring it, photo taken 8/31. 0% water, so the fluid is good

So I left that capful of brake fluid sitting out about 10 days. Here it is on 9/9:

Clearly has absorbed a ton of water from the air.

To be sure, I poured a fresh capful from the same bottle as the first, which I had re-capped normally:

So, my conclusion: all this talk about "trash any unused brake fluid after the bottle is open" is total crap. Re-seal the bottle tightly store it indoors, and you're fine for at least 7 years. Earlier this summer I used similar fluid from 2018 and 2019 on track, and it was fine.

I actually feel much better for having done this little test.

Thanks dude. This is awesome. Also a good way to check that the moisture meter is actually working.

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