Hydraulic brake fluid.

Brandane

Fair weather cyclist.
Location
Ayrshire.
I have Shimano Deore hydraulic disc brakes on my Trek MTB..
Decided today to replace the brake fluid as the rear brake especially felt a bit spongey.
Not being blessed with much patience, I used the only brake fluid which I have in the house which is DOT 4 as used in the car/motorbike.
Does it matter? Will my seals dissolve as I reach a "Stop" sign at 40 mph at the bottom of a steep hill?
 
Shimano use mineral oil not DOT fluid allegedly...

"The seals in the brake are likely to fail with you using DOT fluid, as they are designed to work with mineral oil. DOT fluid will cause premature wear/swelling of the seals in the lever and caliper, neither is a good place"
 

Tim Hall

Guest
Location
Crawley
Two minutes googling suggests that you need mineral oil not DOT fluid. Yes, bad things will happen. I don't understand why people don't check this before doing it.
 

Rickshaw Phil

Overconfidentii Vulgaris
Moderator
One of the things they did on the Cytech course was to show us a few examples of seals that had been exposed to the wrong brake fluid. It's quite surprising how much they can swell up.

Sorry @Brandane you have just knackered your brakes.:sad:
 

T.M.H.N.E.T

Disc brakes - Stopping things since 1902
Location
Northern Ireland
New brakes required..


[QUOTE 4053973, member: 9609"]do you not get much heat build up on disc brakes on push bikes ? I would have thought long down hill ascents at speed would create a fair bit of heat - but presumably not enough to start boiling the oil, hence its use as a hydraulic fluid..[/QUOTE]
Mineral oil is hydrophobic and thus remains stable in it's boiling point of 280oc, DOT fluid is NOT hydrophobic and can vary in boiling temperature rapidly once water has been absorbed. DOT5 is 280oc down to 180oc with just 3.7% volume of water absorbed

PS: Do people actually descend while using their brakes the entire time?
 

Cubist

Still wavin'
Location
Ovver 'thill
New brakes required..





PS: Do people actually descend while using their brakes the entire time?
It's quite useful to do a wee bit of braking on steep offroad descents...which is where hydraulic disc brakes were originally designed and used for. So much so that most of Shimano's MTB range have floating rotors and finned pads available to dissipate heat. All Shimanos use Mineral Oil.
 
OP
Brandane

Brandane

Fair weather cyclist.
Location
Ayrshire.
One of the things they did on the Cytech course was to show us a few examples of seals that had been exposed to the wrong brake fluid. It's quite surprising how much they can swell up.

Sorry @Brandane you have just knackered your brakes.:sad:
After reading the replies on here, I had a dig about in the toy drawer and found some proper mineral oil type brake fluid. So using the vacuum bleeding kit (really for the motor bike but works on all systems) I reverse bled the brakes, sucking the DOT 4 stuff out before replacing with mineral oil. Made sure to run some through the system to try and flush out the bad stuff. Time will tell if it's been successful. Oops! :blush:
 

Drago

Flouncing Nobber
You've just mullered your brakes. Shame, cos well bled with decent pads they're good stoppers.
 

T.M.H.N.E.T

Disc brakes - Stopping things since 1902
Location
Northern Ireland
[QUOTE 4054028, member: 9609"]is the problem with using oil; it does not absorb any water, so any water that may get inside the system would sink to the lowest point (the slave piston) then it could boil at as low as 100c leaving the brakes useless.

somebody very heavy touring with lots of gear, on a long descent could generate a lot of heat in those tiny little discs.[/QUOTE]
I'm pretty sure that brake systems whether mineral or DOT, have been used on modes of transport much heavier than a touring bike in the past.

I feel like you're trying to be another disc brake "anti" but don't know enough about them to do so.

It's quite useful to do a wee bit of braking on steep offroad descents...which is where hydraulic disc brakes were originally designed and used for. So much so that most of Shimano's MTB range have floating rotors and finned pads available to dissipate heat. All Shimanos use Mineral Oil.
It is, we weren't talking about off-road descents though :smile:
 

Rickshaw Phil

Overconfidentii Vulgaris
Moderator
After reading the replies on here, I had a dig about in the toy drawer and found some proper mineral oil type brake fluid. So using the vacuum bleeding kit (really for the motor bike but works on all systems) I reverse bled the brakes, sucking the DOT 4 stuff out before replacing with mineral oil. Made sure to run some through the system to try and flush out the bad stuff. Time will tell if it's been successful. Oops! :blush:
It's worth a try. Had you done a bleed or just topped up? If only topped up you might get away with it but as you say, time will tell.
 

Mobytek

Well-Known Member
Bin em and stat again - DOT will have got onto and into your seals, and the rubber will have tuned to goo - within about 2 minute of contact, so ene romicing the fuild will not bring them back.
 

T.M.H.N.E.T

Disc brakes - Stopping things since 1902
Location
Northern Ireland
[QUOTE 4054224, member: 9609"] (I take it this is similar to Hydraulic Oil used in plant ? [/quote]Yes
(probably sold at 100x the price cause it for cyclists))
About £8 for Shimano Mineral Oil
And also as I have worked a lot with heavy machinery / plant I am aware of the problems with water in such systems. has there ever been an issue with water contaminating these systems on a bike with the resultant corrosion issues and in something like brakes that can get seriously hot, the water starting to boil giving spongy brakes or even complete failure.
Yes, brakes can go "spongy" which is why servicing is available, as are kits to bleed brakes yourself. Disc brakes have been around on bicycles for many years, none of this is new information
 

Rickshaw Phil

Overconfidentii Vulgaris
Moderator
I had done a bleed; but the DOT 4 stuff was probably only in there for an hour tops.
Ah, shame. It probably has done damage then. You'll know soon enough one way or the other.

I recall quite a few years ago taking my car down to the local garage for service and finding they had a Rolls Royce in for work. I was told the owner had made the same mistake as this and that they were having to repair/replace all of the hydraulics (suspension and brakes) at a cost of several thousand pounds just for the parts.:blink: It puts a mix-up with the bike into context.
 
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