Can you use car brake fluid in Hope brakes or best to stick to bike specific Dot 5.1

I’ve used car fluid in the past and don’t remember having any issues. It cheaper too.

Used it for years. No problem.

I’m probably wrong but I thought Dot 5.1 was Dot 5.1 regardless of being sold for car or bike use, apart from being 4 times more expensive when marketed for bikes.

Another happy ‘car’ Dot 5.1 fluid user in Hope brakes.

Is there such a thing as “bike DOT 5.1”? Only problem withe car stuff is it comes in massive bottles and you only need a few ml.

Thanks guys and it is a lot cheaper

Bought the car bottle before after considering the tiny bike bottles to be a rip off. Some argue the stuff doesn’t keep well once opened. Hygroscopic fluid which means it will absorb moisture in the air. That said I’ve had a bottle sitting around for 4 years that had been opened before (used and then closed), and did a bleed recently and perfectly fine.

What they all said…
Silicon based, higher boiling point, non/less corrosive….

I’m probably wrong but I thought Dot 5.1 was Dot 5.1 regardless of being sold for car or bike use, apart from being 4 times more expensive when marketed for bikes.

Its the same with fork oil, put it in a smaller bottle and triple the price and suddenly its bike specific. Surprised we haven’t had enduro brake fluid yet 😆

I’ve got dot 4 race car fluid in all my mtb brakes, there’s no way I’m ever going to boil that!

Dot 5 is the silicone based stuff and not to be mixed with glycol based 5.1 or 4

orangeboy – Member
Dot 5 is the silicone based stuff and not to be mixed with glycol based 5.1 or 4

Listen to that man.
DOT 5 is Silicon based, not 5.1
DO NOT use DOT 5 unless you want to ruin things!
DOT 5.1 & 4 are fine though.

DOT 4 and DOT 5.1 are the standards irrespective of what glycogen them in. I personally use motul tbf600 which is “only” a DOT 4 fluid but seems to offer the best combination of longevity and wet boiling point.

Yep the number is the spec not the pic of a car or a bike. Same with mineral oil for shimano both picked up in halfords when I’m in the UK.

Been using Castrol SRF for years on Hope Brakes. No issues at all.

Aye, Motul RBF600 or 660 (have a motul sponsored motorbike racer friend, so use whichever one I could pinch out of his stock) here when i was running hope and formula brakes, same bottles were used to bleed mtb brakes as the motorbike brakes.

Have switched to shimano brakes on all bikes now, finding the price of the brake fluid to be quite offensive!

The 660 has a higher dry boiling point but due to the lower (than rbf600) boiling point, I should be changed more often.

Having said that, I think both are way over specced for the needs of the average mountain bike.

Its the same with fork oil, put it in a smaller bottle and triple the price and suddenly its bike specific. Surprised we haven’t had enduro brake fluid yet.

Not 100% accurate, you have to get the right weight and RS used to change the weight value so if you got off the shelf stuff you could end up with the wrong weight. Don’t know if the other fork manufacturers are the same.

If I remember RS 15WT is (or was) 20WT Off the shelf stuff.

For the small quantities used, the savings aren’t worth it for me to have a huge vat of oil sat around for years. I just buy the RS stuff, I have a couple of bottles of SRAM oil that have lasted well over a couple of years.

Always seems like the mountain bike paradox, willing to spend thousands on bikes and light compnenets. Complain about a few pounds saving on oil, don’t even mention car park costs 🙂

The 660 has a higher dry boiling point but due to the lower (than rbf600) boiling point, I should be changed more often.

Having said that, I think both are way over specced for the needs of the average mountain bike.

Ironically Dot 4 can have even higher dry boiling points, because the additives used to raise the wet boiling points decrease the dry. Either way, modern brakes seem much better at dealing with heat anyway (phenolic or ceramic pistons, better rotor design, better ventilated calipers) so heat seems to be less of an issue.

Castrol SRF (dot 4) has a wet boiling temperature of 270 degrees, 10 degrees higher than the dry boiling point of standard dot 5.1….which is insane – and it has a dry boiling point of 310 degrees c, only 6 degrees lower than the LCF600 stuff…..

Standard Dot 5.1 has a wet boiling temp of 180 degrees….

The Motul and Brembo oils show marginal gains in dry performance vs SRF and are not worth their marginal gains over 5.1 in terms of wet temperature. With the SRF you can pretty much fill your brakes 6 months before you go out the alps and not care for boiling your brakes – bleed and forget. Even if its relatively old, you will still be getting better performance than your 5.1. Downside is that it’s expensive as hell and as soon as you open the bottle it starts taking on water – so it’s best to share it amongst your riding mates and all bleed your brakes over some beer.

Still I’ve always thought that it’s worth spending money on your brakes and increasing your margin of safety on long extending downhill runs.

Downside is that it’s expensive as hell and as soon as you open the bottle it starts taking on water – so it’s best to share it amongst your riding mates and all bleed your brakes over some beer.

Wet boiling points are given at 3.7% water by volume, so you’d have to put almost a double shot glass of water into a liter bottle to get those numbers. In a closed container it’ll last years without significant degradation. Look at most car (or even a lot of motorbike) reservoirs, the cap isn’t sealed it’s just a plastic lid and it still lasts 2-3 years in the hot and humid engine compartment.

Yup, there is that. It would just pain me to leave a 40 odd quid bottle of fluid lying round when it could be used.

Never tried the Brembo stuff to see if the lower compressibility actually has an effect on lever feel with short brake lines. So don’t know whether it’s worth going with that over the SRF, if you’re not so bothered by the wet temp.

TINAS, totally agree, the mogul rbf 600 is DOT 4 and only because it doesn’t meet the viscosity requirements of 5.1 at lower temperatures. It also comes in 500ml bottles so not too expensive to do the whole fleet. Like you say, the shelf life is pretty good as well due to the wet measures actually needing one he’ll of a lot of water.

Confused I am. All the bottles of 5.1 that I have bought and I look for this, have “Synthetic” written on them….

I’m not wanting to start a Pedant War, but whats going on ?

Synthetic doesn’t mean silicone. DOT4 and 5.1 are made from poly glycol compounds apparently.