Opened brake fluid: problems OTHER THAN lower boiling point?

[d00df00d]

Your call. It is exceptional stuff, but thorough flush is needed. I know some people who use it on track bcs. it allows them to run one season without change, but I know there were issues with it bcs. people didn’t want to flush al the way as it is expensive.

In the M3 I went from BMW brake fluid to SRF without doing that thorough a flush. Pretty sure I got all the old fluid out of the calipers and main lines, but I definitely didn't touch the ABS/DSC module at that time. Zero problems.

Been doing some more Googling and reading. Looks like a lot of people are seeing that SRF is based on "silicon ester" and getting that mixed up with "silicone" (as in DOT 5), and/or they think anything that outperforms DOT 4 by that much must be DOT 5, and/or they are over-generalizing from super-niche race-only brake fluids.

Either way, if there were any potential problems with mixing SRF with other DOT 3/4 brake fluids, Castrol would say something about that, no? They say the opposite -- i.e. that SRF is fully compatible with DOT 3/4 fluids and systems. The only problem they mention with failing to fully flush out the old fluid is that you won't get the best performance.

In theory, I can see the argument that SRF's reduced water absorption might reduce its service life in street use. If left in long enough, it might allow some water to accumulate and pool where another fluid would have just absorbed it. Maybe track people aren't seeing this because the high temps boil off any water that isn't absorbed, meaning the service life of the fluid is dictated mainly by how much water it absorbs, and thus SRF nets a longer service life than other fluids in those regimes even though it'd be unwise to run for too long on the street. IDK. That's some seriously underinformed spitballing on my part.

Either way, the flip side is that SRF would suffer less from sitting on the shelf because it's absorbing less moisture in that time.

So... maybe okay to use if it's just been sitting in a tighly capped bottle, but not for longer than a year in service? Would be interesting to find out. Maybe time to buy a moisture tester and some test strips...

 

[edyvw]

In the M3 I went from BMW brake fluid to SRF without doing that thorough a flush. Pretty sure I got all the old fluid out of the calipers and main lines, but I definitely didn't touch the ABS/DSC module at that time. Zero problems.

Been doing some more Googling and reading. Looks like a lot of people are seeing that SRF is based on "silicon ester" and getting that mixed up with "silicone" (as in DOT 5), and/or they think anything that outperforms DOT 4 by that much must be DOT 5, and/or they are over-generalizing from super-niche race-only brake fluids.

Either way, if there were any potential problems with mixing SRF with other DOT 3/4 brake fluids, Castrol would say something about that, no? They say the opposite -- i.e. that SRF is fully compatible with DOT 3/4 fluids and systems. The only problem they mention with failing to fully flush out the old fluid is that you won't get the best performance.

In theory, I can see the argument that SRF's reduced water absorption might reduce its service life in street use. If left in long enough, it might allow some water to accumulate and pool where another fluid would have just absorbed it. Maybe track people aren't seeing this because the high temps boil off any water that isn't absorbed, meaning the service life of the fluid is dictated mainly by how much water it absorbs, and thus SRF nets a longer service life than other fluids in those regimes even though it'd be unwise to run for too long on the street. IDK. That's some seriously underinformed spitballing on my part.

Either way, the flip side is that SRF would suffer less from sitting on the shelf because it's absorbing less moisture in that time.

So... maybe okay to use if it's just been sitting in a tighly capped bottle, but not for longer than a year in service? Would be interesting to find out. Maybe time to buy a moisture tester and some test strips...

Well, personally, I would shovel tester in. If it stays at 1%, I would go for it.

 

[barryh]

The people saying don't be cheap and just buy new fluid are not answering the OP's question which is a very reasonable one. Fluid doesn't become unusable just because you opened it once.

I have a tin of brake fluid on shelf and the one thing I'm sure of is it's not deteriorating as fast as the fluid in my cars or motorcycles brake system. That says to me it's probably good for at least 2 years on the shelf and I will use it without concern over that timescale.

My inclination is not to buy very high performance racing brake fluid as there is usually a trade off of it needing to be changed more often and it's performance is usually not needed anyway. While I'm not familiar with SRF, it's generally accepted that high performance DOT 5.1 absorbs water faster than DOT 4 which in turn absorbs water faster than DOT 3. If the cars manufacturers recommendation is DOT 3 that would be good enough for me. The only reason to use anything with a higher boiling point would be if the brakes are going to be used hard e.g. racing.

 

[edyvw]

The people saying don't be cheap and just buy new fluid are not answering the OP's question which is a very reasonable one. Fluid doesn't become unusable just because you opened it once.

I have a tin of brake fluid on shelf and the one thing I'm sure of is it's not deteriorating as fast as the fluid in my cars or motorcycles brake system. That says to me it's probably good for at least 2 years on the shelf and I will use it without concern over that timescale.

My inclination is not to buy very high performance racing brake fluid as there is usually a trade off of it needing to be changed more often and it's performance is usually not needed anyway. While I'm not familiar with SRF, it's generally accepted that high performance DOT 5.1 absorbs water faster than DOT 4 which in turn absorbs water faster than DOT 3. If the cars manufacturers recommendation is DOT 3 that would be good enough for me. The only reason to use anything with a higher boiling point would be if the brakes are going to be used hard e.g. racing.

In England, no. Maybe in Arizona or Colorado. Maybe.

 

[Faawrenchbndr]

Or just put the old fluid back in the bottle, clean it up real nice and take it back to Walmart for a refund. Which is what some guy did before me and I am the lucky one who bought it!

Been there, done that……

I check all for the foil seals now. Sad but that’s where we are now days

 

[Faawrenchbndr]

The people saying don't be cheap and just buy new fluid are not answering the OP's question which is a very reasonable one. Fluid doesn't become unusable just because you opened it once.

I have a tin of brake fluid on shelf and the one thing I'm sure of is it's not deteriorating as fast as the fluid in my cars or motorcycles brake system. That says to me it's probably good for at least 2 years on the shelf and I will use it without concern over that timescale.

My inclination is not to buy very high performance racing brake fluid as there is usually a trade off of it needing to be changed more often and it's performance is usually not needed anyway. While I'm not familiar with SRF, it's generally accepted that high performance DOT 5.1 absorbs water faster than DOT 4 which in turn absorbs water faster than DOT 3. If the cars manufacturers recommendation is DOT 3 that would be good enough for me. The only reason to use anything with a higher boiling point would be if the brakes are going to be used hard e.g. racing.

Unless the brake fluid is completely sealed, and essentially vacuumed of air, it will microscopically gain moisture, i.e. water, content. In my eyes, that makes it unusable……period, the end.

Once opened, I do not reuse it. It gets recycled. It just isn’t worth it.

The question has been answered.

 

[d00df00d]

Unless the brake fluid is completely sealed, and essentially vacuumed of air, it will microscopically gain moisture, i.e. water, content. In my eyes, that makes it unusable……period, the end.

Once opened, I do not reuse it. It gets recycled. It just isn’t worth it.

The question has been answered.

With respect.... The main question was whether there were risks to opened brake fluid other than moisture lowering the boiling point. @ctechbob answered that. No one else even tried.

The unsealed-but-capped bottle of unused Castrol SRF I have right now was opened ~7 months ago. I just stuck a brand new Cal-Van 66 brake fluid test pen in it. The reading was 0%.

The SRF currently in our M3 is partly ~7 months old (hence the unsealed bottle) and partly 30+ months old. Again, 0% on the test pen.

 

[d00df00d]

The unsealed-but-capped bottle of unused Castrol SRF I have right now was opened ~7 months ago. I just stuck a brand new Cal-Van 66 brake fluid test pen in it. The reading was 0%.

The SRF currently in our M3 is partly ~7 months old (hence the unsealed bottle) and partly 30+ months old. Again, 0% on the test pen.

Follow-up to this as the edit window has closed.

The Cal-Van 66 has 8 settings for different brake fluids: generic settings for DOT 3, DOT 4, and DOT 5.1, plus individual settings for 5 specific DOT 3 fluids. I tried all the settings in the unused SRF and they all showed 0%. Only used the generic DOT 4 setting in the M3's reservoir.

Felt I should include that given the potential unreliability of these test pens.

 

[Faawrenchbndr]

With respect.... The main question was whether there were risks to opened brake fluid other than moisture lowering the boiling point. @ctechbob answered that. No one else even tried.

The unsealed-but-capped bottle of unused Castrol SRF I have right now was opened ~7 months ago. I just stuck a brand new Cal-Van 66 brake fluid test pen in it. The reading was 0%.

The SRF currently in our M3 is partly ~7 months old (hence the unsealed bottle) and partly 30+ months old. Again, 0% on the test pen.

Ok……you do you.
I’ll just buy what I need and and recycle old and excess.