[Closed] Where does the air come from? Brake throw and feel

its to do with the seals on the pistons. Basically the seal deforms and pulls the piston back after use. this is how your brakes retract. It doesnt get sucked back or anything like that. if you tie your leavers up it allows the piston to slide and the seal to recover a bit so your pistons are effectively closer to the disk. This reduced the dead spot on first pull. As you use the brake the disk may push the pistons back slightly increasing the dead spot and leaver throw.

I've never had that problem once brakes were properly bled. They can often be difficult to bleed.

This is a schematic of how master cylinders work. The fluid flow is complex and it's easy for air bubbles to get trapped and then released later when you turn the bike over. If air is actually getting into the system, it would be through the secondary seals. That's not going to happen with every single brake though.

https://www.accessnorton.com/attachments/5-classification-of-brakes-28-638-jpg.70136/

its to do with the seals on the pistons.

This won't cause the symptoms the OP describes. His symptoms sound like air in the master cylinder. Possibly a leaking secondary seal, but probably through not being thoroughly bled. Brakes can be bastards to bleed and everyone who's tried has had the experience of thinking they were properly bled but finding there was still air in the system.

i disagree

Another with the view its pistons at the caliper.

Bet when they're left clamped on over night, the pistons shift a bit in the seals and become nice and square with the rotor, then after multiple pulls, they're no longer perfectly square to the rotor, hence feel goes to pot since now you're flexing stuff because it's nonlinear all square.

It's like 99% of the time when people say they need a bleed, they don't, it'll be caliper/piston alignment and/or sticky piston(s).

Haven't anyone else noticed 2 piston brakes are a lot easier to keep feeling good than 4 piston brakes? more scope and likelihood of misalignment and sticky pistons with 4.

I m with andy brad - You are not removing any air by tying the levers on - you are resetting the pistons

If you pull the lower syringe hard you'll get dissolved air coming out of the fluid under the lower pressure/partial vacuum.

I'd think given the cycling of pressure and release some of this will come out of solution.

I keep meaning to try a bleed with fluid that has been subjected to a lowering of air pressure to see if it makes any difference.

Agree with @onehunderdthidiot. Leaving your brakes on overnight increases the pressure in the system and dissolves air into the fluid. Your brakes feel great. You go for a ride or two and all that fluid mixes with the fluid in the reservoir and all the gas comes out of fluid and you get a bunch of microbubbles that give you a slightly spongier lever.

De-gassing the fluid is key before bleeding and I'm surprised it isn't covered more in official bleeding guides. Some do mention it. I also think this is more important for DOT 5.1 fluid than mineral oil.

I've just bled my SRAM Guides with a brand new bottle of Mobil DOT5.1 and the amount of air that I got out in the syringe by vacuuming it slightly was insane.

If you pull the lower syringe hard you’ll get dissolved air coming out of the fluid under the lower pressure/partial vacuum.

I’d think given the cycling of pressure and release some of this will come out of solution.

I keep meaning to try a bleed with fluid that has been subjected to a lowering of air pressure to see if it makes any difference.

SRAM instructions make a big fuss about degassing the fluid before bleeding. By contrast Hope most people just pour fluid into an open reservoir. I've generally had less trouble getting a good bleed and consistent feel on Hope than SRAM, though SRAM have been a lot better since I ignored their precise procedure (now - push dirty fluid from out of calliper by pushing from lever end, remove and discard the dirty stuff. then start flushing with clean fluid and do it back and forth, and using vacuum at either end)

IME the best way of getting all the air out of any nooks and crannies where it was doing no harm is to put your bike on a trailer and bounce it up a mountain track. That normally ensures anything less than a perfect bleed will lead you with shit breaking on the first big descent of a holiday.

I think tying the levers helps get air out of the master cylinder. If you aren't careful about getting the reservoir full of fluid with no air in there, air can get into the mastercylinder if you lay the bike down. Also, when the pads wear, there won't be enough fluid in the reservoir so that makes it easier for air to get into the master cylinder. When you clamp the lever, the bubbles are compressed and it's easier for them to float to the top of the system. Then, when you release the lever, fluid is forced back into the reservoir and pops the air bubble back into the reservoir.

Air cannot get into the system through the caliper. If the seals were that leaky, fluid would leak out when you applied the brakes. If fluid isn't leaking under pressure, then air can't leak. Air can get in through the secondary seals in the master cylinder because that's a low-pressure area. It can also get in if the seals in the reservoir are damaged.

If the OP is finding that his brakes are always becoming spongy, it's almost certainly a problem with bleeding - leaving air in the reservoir, probably. Brakes can be tricky to bleed and getting all the air out can be really difficult. Everyone who's ever bled brakes has had the same experience of swearing that there cannot possibly be any air left, only to find that there actually was.

De-gassing the fluid is key before bleeding

It won't cause the brakes to suddenly become spongy. It doesn't matter if there is air distributed in tiny bubbles or they merge and form big bubbles, the volume of air is exactly the same. If the brake has a firm lever after bleeding then the fluid doesn't need de-gassing. I've been bleeding car brakes and then MTB brakes for 30 years and never de-gassed fluid, never been a problem.

That may be true, and I bow to your much greater experience on the subject. But surely if a solution starts off with less air dissolved in it, it can accommodate more air before bubbles are created? So any slow air leak into the system would be accommodated by a de-gassed fluid, up until some critical time point when the user can feel it. That usable life might be shorter if you didn't de-gas the fluid in advance?

FWIW whenever I've de-gassed DOT fluid, you can force a surprising amount of air out of it, but it almost all goes back into solution when you release the vacuum. I always wonder how effective this procedure is.

So any slow air leak into the system would be accommodated by a de-gassed fluid

Air can't leak into the system. It can only get past the seals if there's a pressure difference. The air on the outside is at atmospheric pressure. The fluid on the inside cannot drop below atmospheric pressure but can be far above atmospheric pressure when the brakes are applied. If air can leak into the system, fluid will leak out when you apply the brakes.

The exceptions to this are the secondary seals in the master cylinder and the diaphragm in the reservoir. If air is getting into the system, these are the only plausible ways in. Otherwise, it's just that they hadn't been bled properly and there was air in the reservoir all along.

I do de-gas the syringes - both of them - spend almost 10 minutes doing this and ensure all the bubbles are at the top of the syringe and I then push the plunger down to remove the bubbles.

I'm happy to do another bleed on the system, I'm just unsure how it can go from a nice firmer lever feel to a far squishier feel in one or two rides - it isn't lever back to the bar, just an increase in lever throw more than squidge.

I'll try and rotate the lever and caliper around as I'm doing the next bleed to encourage an easier transfer for any air in the system...however, I do think my bleed process is decent (clearly not if the lever throw increases though).

OP I have the exact same issues and was pondering this very thing. Bike stored by hanging from it's front wheel, brake lever strapped on ...great rear brake feel next day.

I subscribe to the haven't bled it properly opinion and the piston seal effect.

Post again if you crack it !

I always "de-gas" when bleeding my srams, brake still has the OP's symptoms, I don't think dissolved air is a cause.

I also have put the bike upright overnight with the brake on, so any dissolved gas will come out the solution, and should rise up to the lever end, then pump lever with the lever still at the highest point to encourage any bubbles into the master cylinder/reservoir, then bled the reservoir, not convinced I've ever got out anything other than the bubble introduced by fitting the bleed syringe.

On a side note, brakes do feel better after de-gassing the fluid.

I have never degassed brake fluid.   symptoms sound like maybe air in the reservoir so when the bike is stood up or on its side air enters the system from the reservoir