In the context of bleeding brakes, why do people recommend putting an elastic band on the brake lever overnight?

it opens up all the valvey bits in the master cylinder and allows the air to float up to the top of the resevoir, where its less of a problem? (iirc)

ive done it before, but with a zip ties.
brakes feel much better in the morn ususaly.

Good for Juicy’s in particular, apparently…

Well it doesn’t allow the air to float up to the top (or it would do it as you applied the brakes), apparently it forces it in to the fluid whence it could appear again as a bubble at a moment of dire consequence – or so the experts tell us.

How would it rise to the top against the pressure of the system being applied when it won’t rise without that pressure?

By using the brakes whilst riding, they don’t remain ‘open’ long enough to allow the air to move…by leaving them overnight (with the reservoir as the highest point of the system), you give the air enough time to work it;s way up the hose to the reservoir. This doesn’t get rid of all air but gets rid of a huge chunk.

Thing to be aware of is Dot fluid is hydrascopic so rather than leaving the top cap off and the fluid exposed, I tend to lock the lever down and in the morning, remove the top cap, then release the lever…normally if I have air in the system I can see a drop in the fluid level…which I then simply top up and put top cap back on.

Sort of the same reason why reverse-bleeding works well – it forces the air to move along with the flow of the fluid and ensures the air is forced up and out the system (air rises naturally).

Orange Crush – Member
“How would it rise to the top against the pressure of the system being applied when it won’t rise without that pressure?”

It’s not rising agains the pressure of the system, pressure doesn’t have a direction.

When you press the lever you are SHUTTING the resevoir port!

I think what happens is that the seal in the master cylinder takes a set in the bore(full diameter)over night due to the pressure and the seals V section shape and so when you next use the brake it is pushing fluid right from the off where it had been passing down the bore a way before the pressure built up enough to make the seal open up to full diameter and so seal 100%.
Result is a much better brake but I don’t know if any air is removed.

Dot fluid is hydrascopic

Hygroscopic.

It just resets the caliper pistons a little closer to the disc (piston seal is also the retraction spring )
“it opens up all the valvey bits in the master cylinder and allows the air to float up to the top of the resevoir, where its less of a problem?” 😆 Ta

It’s magic?

It doesn’t matter how it works – it just does.

I’ve done this for years. If I’ve bled brakes or even put new pads in I’ve stuck a business card either side of the rotor and zip-tied the lever back over night.

It does just seem to reset everything and get the pad spacing nice.

It does work, although it only seems temporary in my experience, by the end of a ride the travel is back in the lever. Best way is a proper bleed

I thought it was because the extra pressure compressed any air bubbles making them small enough to rise up the hose easily.

Either way it works, especial if after bleeding the levers feel ‘different’ either one firmer than the other or different travel to the bite point etc………

My own understanding- and this could be mince, it’s just from observation- is that it helps the bubbles to gather at the lever. When you release it, that opens the ports and allows the gathered air to escape into the reservoir (which is why it can take a pump or 2 to get the benefit). But, that doesn’t get rid of the air, a reservoir full of air is still a recipe for air in the working fluid- this method works brilliantly for motorbikes where you have a pretty stable relationship between all the parts, but it seems to be less effective on a mountain bike thta gets turned upside down, crashed into trees etc.

I thought it was because the extra pressure compressed any air bubbles making them small enough to rise up the hose easily.

Won’t the pressure make it worse? Bubbles (of compressible air) in the non-compressible fluid will be less buoyant the smaller (and more dense) you make them. Bit like using a vacuum pot to de-gas resin or silicone before casting with it?

possibly, but the difference in density is still going to be in the order of 10’s or 100’s, but your reducing the surface forces between the air/liquid but reducing its volume making it more likely to move.

There’s some pish talked on here int there…. 😀

indeed, got a theory?

either way it works 🙂

An open system is only ‘open’ when the lever is open. When you pull the lever, the system closes (otherwise, the brakes wouldn’t work). The point of an open system is to allow air bubbles to leave the pipes (works best with the lever/reservoir at the hghest point), and to ‘add’ more fluid to the system when the pads wear.

So, however (if ever) tying the lever back works, it’s NOT by allowing the bubbles to escape (as someone else mentioned!)

I’m thinking about making a vacuum bleeding system – I saw it on discovery channel (a bike is born!) and it seems a really good idea – you literally suck the old fluid out of the bleed nipple, topping up at the resevoir as it empties…

DrP

When I’ve had to resort to tying the lever back, I will then bleed the brakes. Its a quick fix and doesn’t need doing all the time.

I bought a pressure bleed kit to do the clutch in my car at the weekend, forces fluid through using pressure from the spare tyre (mmm, black dot fluid!). Has a variety of reservoir caps but it says if none fit, just drill a hole in the cap and then attach the supplied nut and washers. £20 well spent.

So I’m wondering about buying a spare cap for my hopes as it was much easier than all the squeeze lever tighten nipple release lever open nipple malarky. Will just have to block up the tiny breather under the cap temporarily.

Spooky, you can buy the correct hope cap/seal all pre-drilled from Hope as they sell a similar pressure bleeding kit.

DrP – Member

“I’m thinking about making a vacuum bleeding system – I saw it on discovery channel (a bike is born!) and it seems a really good idea – you literally suck the old fluid out of the bleed nipple, topping up at the resevoir as it empties…”

Syringe will do it. I’ve fannied around with Mityvacs and the Sealey pressure bleeder (which turned itself into a brake fluid pressure washer, nice) but syringes work better, they allow both suck and push which is ideal. Also very cheap which is ideal 😉

Thanks coatesy!

What is the internal bore of the hydraulic brake pipe? I reckon 2mm. I simply can’t believe that an air bubble would rise up such a narrow pipe against the viscosity of the fluid.

Years of messing with bike and Land Rover brakes tells me that bleeding fresh fluid through until it emerges bubble-free is the only answer. Brake pistons move thanks to the flexibility of the seals and it could be that the rubber band trick allows the pistons to creep through the seals a little and take up some pad wear.

I’m going to have to try this now as I hate bleeding brakes and avoid it at all costs
I don’t care how it works, just that it does
ta 🙂

globalti – Member

“What is the internal bore of the hydraulic brake pipe? I reckon 2mm. I simply can’t believe that an air bubble would rise up such a narrow pipe against the viscosity of the fluid.”

That’s great. So what do you think is happening when you take a spongy back-to-the-bar lever, strap it back overnight, then in the morning give it a pull and find it’s working?

If I’ve shortened hoses, or done something else to get a bit of air in the system, a zip tie to the lever overnight always works for me. A couple of taps when the lid is off in the morning you see the tiny bubbles popping up to the surface. Done it for years – works everytime.