My bike is a 2016 Enduro, so probably manufactured 2015. Its only had a dozen or so rides in that time but that’s changing as I begin to get out more.

The brakes are Guide Rs and I experienced real ‘pump up’ of the brakes on a techy section of tract – to extent that the rear wheel just wasn’t turning! It freed up but I do wonder if the fluid just needs changing after all that time?

A bleed seems the way forward but am I guessing correctly on the fluids going off in some way or is it likely another issue? Pistons don’t appear stuck in any way.

Dot fluid absorbs moisture.

Over time this lowers the boiling point, if your issue is happening under heavy braking then this sounds like exactly whats happening to your brakes.

Give them a bleed with fresh fluid from a sealed container and see if it improves

The DOT fluid in your brakes does take on water over time but I very much doubt that is what you are experiencing.  I think your problem is SRAM brakes and a bleed should sort it.

Edit: Pipped to it by a few seconds^

If it’s absorbed water you’d get the exact opposite effect. The water would boil, turn to steam (which is compressible) and you get a spongy lever which would return to normal when the brakes cool down.

Dot 5 fluid is not hygroscopic and avid/SRAM brake fluid is dot 5.

DOT 5.1 – very different.

@Wobbliscott,
No.

SRAM use DOT 5.1, which is a higher performance version of DOT 4, which they were probably using in 2015. So it IS hygroscopic.

DOT 5 is completely different chemically and NOT compatible with SRAM brakes!

https://epicbleedsolutions.com/blogs/faq/whats-the-difference-between-dot-4-and-dot-5-1-brake-fluid

Dot 5 fluid is not hygroscopic and avid/SRAM brake fluid is dot 5.

No it is not. It’ll be Dot 4 or Dot 5.1 – both of which are “normal” brake fluid – so absorb moisture.

Dot 5 is silicon-based and doesn’t absorb water. But it shouldn’t be in your brakes.

Edit – as all of them said faster than I did ^^^^^

Blame the Dept. of Transportation for their stupid naming scheme

@CB,

I have had the same issue with older Avid/SRAM brakes. Pads pushing out in hot weather. Luckily I have had it on brakes with contact point adjustment, so I could simply dial that in, to solve the issue.

For sure you should bleed your brakes, (once a year is good for DOT fluid brakes) but I did that to the brakes on my 9 year old daughter’s bike (my wife’s old bike) recently, and it doesn’t seem to have solved the problem.

My newer SRAM brakes have never done this, so might be time for an upgrade. Shimano Deore/SLX are very good too.

My brother has an old bike with black hope m4 brakes with the gold caps, hardly ridden and collecting dust, brakes haven’t been bleed since 2013 but still work, the seals at the lever definitely need replacing soon as they’re looking dry n crusty, God knows how the piston seals are still doing their job.
The fluid will probably be black so new seals and complete flush through with be my next job.

As above, I’d expect boiled fluid to result in mushy brakes or the lever pulling to the bars.

The brakes should accommodate fluid expansion by allowing fluid back into the reservoir. If that’s not happening it suggests that the port to the reservoir is blocked or that the master cylinder isn’t retracting properly and clearing the reservoir port.

My now pub bike has avid brakes that haven’t been bled in my 15 years of ownership. I was under the impression that its a sealed system so shouldn’t be able to absorb moisture. That’s not saying they don’t need bleeding but they are working fine.

You didn’t boil your brakes even with wet fluid unless you were dragging down thousands of metres of huge fast alpine descent.

It could be something like corrosion around the seal making it a bit too tight, and then as the brake got warm it made it too tight to flex properly.

Or something like what pdw says. I’d be replacing caliper seals, cleaning out any corrosion and refilling.

I reckon that you’ve got there guv, is the classic Guide swollen piston.
Is the lever a little lazy to return if you let go?

It is a bit lazy actually. Swollen piston? Maybe back to sram for service. Last night’s ride it didn’t happen, seems to be only on real stop, start stuff. Thanks all for responses

Iirc there was an issue with the seals in the master cylinder causing the brakes to lock up under prolonged braking on descents when they heat up.

Does brake Fluid ‘go off’?

Considering the nature of it’s application, if there genuinely was a shelf life, i think it would be printed on the bottle and fairly prominently.

Can you damage it thru miss-use
probably, details will be on the bottle or data sheets / website.

I think that would be my general expectation re brakes.

yes it does go off as it absorbs moisture from the atmosphere. This can then crease issues with corrosion in the system and this is often why its advisable to swap it out on your car every couple of years.

youll also get stuff dragged back through the seals as they are never 100% sealed.

yes it does go off as it absorbs moisture from the atmosphere.

DOT fluid does, mineral oil (Shimano etc) doesn’t.

yes it does go off as it absorbs moisture from the atmosphere. This can then crease issues with corrosion in the system and this is often why its advisable to swap it out on your car every couple of years.

It’s a bit more complex than that. DOT fluid absorbs water and the boiling point decreases. However, bike brake systems are sealed, so they actually run for years without any problems. If you’re racing DH or doing crazy descents in the alps, you would definitely flush the system with fresh fluid just to be sure, but the average rider is not really doing to have a problem unless they’ve thrown their bike into a swimming pool and left it there for a month.

Mineral oil based fluids don’t absorb water like DOT fluid. However, if water does get into the system, it will pool at the lowest point (the caliper), whereas it will end up distributed throughout the system in a DOT based system. The caliper is the only part that actually gets hot, so mineral based systems end up with the water at exactly the worst place. The claim that water is a problem for DOT, but not for mineral oil just doesn’t really have any factual basis – it’s not generally a problem for either, but if it does get into the system, it’s a bigger problem for mineral based systems.

I agree.

however the seals by their function allow small amounts of fluid to escape and thus the potential for contamination.