Why do some videos suggest tying up your brake lever while bleeding

I read somewhere that ultimately all's your doing is pressurising the system so your forcing the air back into the fluid. Pulling a vacuum on the syringe has the opposite effect. So that does make sense

Your just delaying the inevitable bleed that is needed so not worth doing if you ask me.

I read somewhere that ultimately all’s your doing is pressurising the system so your forcing the air back into the fluid

That's pretty much exactly my thinking but I'm seeing this advice in videos.  I'm wondering if it is a hangover from a different system as has now become a piece of accepted lore without really making much sense.  I do see in the Park Tool video they put a band on the lever but that is only for a very short time so they can push oil out of the port on the piston, then they remove it.

Pulling a vacuum on the syringe has the opposite effect

Exactly that as well.  When I worked on high voltage stuff we used to pull a vacuum on the oil while running it over a surface to pull any air or moisture out.

Putting a band on the lever maintains compression throughout the system and generally forces micro bubbles, that tend to cling to surfaces, to join together creating a bigger easier to bleed out bubble. Vacuum bleeding achieves a similar thing in that reducing pressure allows the micro bubbles to grow and so are easier to bleed out. Both systems work and one can work when another doesn't for stubborn systems

thanks, that makes more sense than the system being open so bubbles can get out

Well ive been servicing a moto v2 last week, replacing the caliper pistons.

For bleeding, obviously the system was empty. I filled up, open,pump, close etc etc, but filled the master cylinder then went to make a cuppa, and came back to find the reservoir empty. because when the lever is in the open position, the fluid can transfer from the lever to the caliper, so im not sure what difference having them lever tied on would make.

Pressure testing is the system closed, pads or whatever to stop the pistons moving out, then pull and tie the lever against the bar and leave it to see if the system is sealed and no leaks from anywhere. Which i did with the moto V2 and left it 24h and thankfully no problems.

I'll just stick to then open-pump-close technique. Works for cars and M/bikes,so why mess with it.

Are these official Shimano videos recommending this? If not, less of a concern of being correct.

As above, I think it allows micro bubbles to migrate to the top. When you open the lever, they can pop back into the reservoir.

With Shimano brakes with the funnel, rather than doing this I just squeeze the lever as hard as I can for 30 seconds or so then release, repeating until bubbles of air stop popping out the top of the reservoir. Even if you bleed thoroughly, it's still easy to have a few small bubbles in the system. The air will collect in the reservoir and the brakes will work perfectly until you lay the bike down and the air will get into the master cylinder. Then you'll get the wandering bite point problems until the air works its way back into the reservoir again.

I’ll just stick to then open-pump-close technique. Works for cars and M/bikes,so why mess with it.

This. Works on every brake (bar my rear V4 but I have my suspicions about that) I've ever bled and given a solid result.

I don't know what it actually does, but it does work. I've always assumed that pressurising the system encourages air to move out of the places it gets stuck, then being air it slowly floats up, so that when you remove the cable tie/whatever, it can then escape into the reservoir. Course, you don't want air in the reservoir, but if you've got air you'd rather have it there than the piston or hose.

Classic motorbike trick but works best if you combine it with a car journey, bouncing around on a trailer really gets the air out.

I don’t know what it actually does, but it does work. I’ve always assumed that pressurising the system encourages air to move out of the places it gets stuck

Yes, when you pressurize it, the bubbles are compressed to a fraction of their original size so they can move around more easily. I find repeatedly pumping the levers as hard as I can help air bubbles work their way up to the reservoir, think having them compress and expand jolts them into motion.

I’ll just stick to then open-pump-close technique. Works for cars and M/bikes,so why mess with it.

I've had a motorbike and multiple cars where this doesn't work. Complicated routing on a moto rear brake and over a bulkhead on a '60s car meant little pockets of air made it really awkward. Fast pumping the car brake helped with the car. Bought a vacuum bleeder for motor vehicles - super quick. Pedal bikes i've never had a problem with, but sometimes have to do the rotate and flick the lever thing to get it right.

Are these official Shimano videos recommending this?

No, it's never in information from sources like Shimano, park tool etc. But you do see it on other people's videos.  I've always done the compress and quickly release the lever thing and that has worked for me.

My main worry was that had misunderstood how brakes work but it's clear I do understand and that compressing the lever is helping bubbles come out of suspension.

Now it's got me wondering if I want to use my vibrating multi tool against the side of the cables to help move things along.  Must..stop.. overthinking.

I can bleed brakes just fine but the lever tying thing made no sense

What have I missed?

Doing the tying thing after you've completed the official bleed thing (or is that what you're doing?), it seems to move the last gnat's lungful
For brakes with a horizontal hose along the chainstay, get the front wheel up overnight as well and tap the hoses for a few minutes to encourage bubbles to move
Get the bike level the following morning and work both brakes (opens and closes the port) then top the reservoir up

I still reverse bleed (syringe of clean oil into the calliper).

That gets a high flow rate of bubble free oil into the bottom of the system, disturbing and pushing upwards any bubbles - I think the flow rate will be faster and more sustained than pumping the lever, and I think it should help disturb any stubborn bubbles.

Either way, Hope brakes and never had an in satisfactory bleed (though Hope do seem to just about bleed themselves)

I still reverse bleed (syringe of clean oil into the calliper).

That's exactly what I do now and it seems to always work.  I was just confused as to why tying the levers overnight did anything at all

I still reverse bleed (syringe of clean oil into the calliper).

Reverse is standard for both SRAM and Shimano anyway. I go top to bottom first though - it's the oil at the bottom/calliper that gets really dirty (I've had it come out black after a few weeks of uplift) and I reckon much better to get that out of the calliper rather than push it up through the lever. Once I'm seeing clean fluid at the bottom I reverse and bleed upwards.

I still reverse bleed (syringe of clean oil into the calliper).

It depends on the geometry of the caliper/lever, if there's a void in the caliper (i.e. the space behind the piston) and the bleed port is at a high point then all you're doing is dribbling fluid through that air pocket, around the caliper and back out the hose on the other side, likely entertaining air as you go. That's why a lot of instructions tell you to push/pull on the caliper syringe several times, it's to flood the caliper with fluid before you start pushing fresh fluid up into the lever.

The problem with bikes is the rear caliper is often 90deg to the front one, so the ports in the caliper will rarely be where you need them to be. Which is why some instructions recommend taking the rear caliper off or mounting the bike vertically in the stand.

There's some quirk with SRAM brakes that seems to mean unless you follow their instructions to the letter, it doesn't work. So I usually do that, then bleed the lever again after a few days to eliminate any air that's moved up into the reservoir bladder.

There’s some quirk with SRAM brakes that seems to mean unless you follow their instructions to the letter, it doesn’t work.

I've got much better bleeds since I stopped following their instructions to the letter!

Push out dirty fluid at bottom
The push clean fluid up to top
Push pull for a while.
Shut top syringe.
suction on caliper
Pull lever, push fluid through as slowly release
remove bottom syringe and close bleed valve.
Alternate pressure and vacuum on lever a few times (I'm always amazed how often this still brings out air after you've done the rest)

Isn't this just after the actual bleed - and it allows all the little air bubbles to congregate into one under pressure. This then makes whatever is holding the brake lever 'on' go slack and indicates that a bit more bleeding is required. And makes that bleed easier because there aren't hundreds of little bubbles clinging to bits of the system.

If you think about it, this is what you are doing when you drag the brakes down a long descent - and adding heat into the equation too. The sudden pooling of lots of inconsequential air bubbles into one big one is what causes the sudden pull back to the lever and loss of power. This is especially true when you ride somewhere 'bigger' than your normal patch. The likelihood of having the brakes on longer to pool the air bubbles is higher.

Isn’t this just after the actual bleed – and it allows all the little air bubbles to congregate into one under pressure

Yep, it's not during the bleed, it's a last step as you said.   I'm assuming you have already turned the bike on end or removed the calipers etc.

I think I might leave a syringe full of oil under pressure overnight to see if any micro bubbles appear anywhere

Alternate pressure and vacuum on lever a few times (I’m always amazed how often this still brings out air after you’ve done the rest)

I think that's because the bleed port isn't at the top on the lever, so unless you do that step to pump the air out of the internal bladder/diaphragm.

Which is why I think the 'official' method works (for me anyway), but deviations from it don't. And also why they have the vacuum step, there's expansion room in the lever, but there isn't a reservoir vertically above the cylinder as such for the air to escape into.

Dunno why it works but it does. Do it on the single circuit in my mini as well. Just with a bit of wood instead of a rubber band.