How To Clean Hydraulic Brake Pistons - Tech Tuesday  

I think they're simplifying the explanation here. Also have to remember the oil reservoir on the master cylinder, its open to the cylinder at the resting position, but closed off during motion. This means as the master cylinder returns, just before returning to rest the system becomes open and oil pressure is relieved; so the slave pistons don't get fully sucked back to where they started. At the same time, the flexing seal means there's preferential for the slave pistons to slip more in one direction than the other. Both things need to work together for the pad wear compensation.

@@adriansue8955 yes thanks so it is a two sided story.. The pressure side and the relaxing side. Nice to get more understanding

@@adriansue8955 I think the way it works, is that the seal can also flex a bit in the other direction. Only when the master cylinder piston returns past the reservoir timing port, the caliper piston seals can relax to the middle position.

I never knew this!! Amazing and extremely underrated comment.

for me its more like, I've never seen an explanation for why they should want move in synch in the first place. There's no mechanical coupling between them, and fluid's going to take the path of least resistance. So in my mind is a miracle that they'd even be as close as they are.

@@adriansue8955 the fluid isn't taking any paths of least resistance, as the resistance is the same on both sides of the pistons. Theres the same amount of gap for the fluid behind each piston. So whats happening is even pressure is being applied to each of the calipers, its just up to the seal to bring it back after pressure taken off,

@@akeluify "the resistance is the same on both sides of the pistons" ...in lab conditions when both pistons are brand new. And even then there will be differences resulting from how each seal interacts with each piston. Once they start picking up dirt these differences will increase and each side will retract differently.

Just did on my 4 pistion SLX brakes...I hope my feel as good on the trail as they do in the garage

(glad it helped but) isn't that against SRAM manufacturers guidance, to NOT use dot fluid around the piston? I am wondering if the fluid would make the seal slide instead of GRIP and pull back, the way it should....? The reason is, I have pistons that don't retract enough after use, so they grip juts enough to stop the wheel spinning freely ...

Funny guy! :))

I came to the comments for this

Lol wanted, to write same thing ;)

..that is ethanol !. Surely a cheaper and better option would be isopropyl alcohol and save the ethanol for a G&T?

Could i also use regular brake cleaner or lighter fluid for the cleaning part?

I'm also in the camp of after the cleaning process to remove excess oil with brake cleaner or alcohol so that dust won't collect.

Agree, this will prevent unintentional oil transfer and pad/disc contamination.

Guess the bike is set up for a left hander. Ha! Good thinking.

Countries that drive on the right have front brake on the right. Opposite for GB AU NZ SA( and probably HK and JP...,)

Don't bet...try it. You'll be 100% right. With 4 piston pucks its even whackier

I'm no expert so take this with a grain of salt ... one issue is that the brakes can get really hot this will cause oil and grease to become more viscous and travel to where you don't want it.

And cheap imprecise engineering. It's plastic bike parts as opposed to high end aeronautical parts. ? My guess only.... But yes, not ideal.

Dude, that is dank

Dude, this is nice to know when you have that :))

Nice, you have eyes

Oh the good old days of no social distancing. Thanks for watching!

So these guys are wrong? I think the purpose of the spring is so the pads don't rattle, similar to a car.

@@stayathome2 the pistons move more than just the roll back from the seals

clean first, bleed afterwards ( that way dirt and gunk will be out of the system after seal cleaning )

Afterwards as N Benci said.

@@parktool thank you, I thought so.

Check out this video for more on adjustment. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-uk_nC9anQcM.html&t

If you have something like a Hope caliper, then you can purchase seals and pistons to service your caliper and return it to as-new. Sadly, for a Shimano brake like this one, they don't sell service parts separately so eventually you'll have to chuck it in the bin and buy a new one. Booo Shimano :/

Does the lever go to the bar or is the power just not there?

Look to see if there is a rebuild kit for your brake. The seals can go bad over a long period of time.

Great questions! Typically the reservoirs have a bladder that takes up the space that the missing fluid would have taken up. These bladders also act as a seal. As pads wear the advancement of the pads can get to the point where all the fluid is used and you will have a "dead" lever. It will start to get closer and closer to the bar. This is typically a good time to of course replace the pads but often to bleed the system.

@@parktool I see! Thanks very much indeed for the quick reply. So you know that it is appreciated, it does influence my choice of tools and I do try to give back via purchases when possible. Sorry to add further - I’m almost there but not quite. I think the bladder is the same as the diaphragm, without which the whole sealed unit would be solid and you wouldn’t be able to press the brake lever (apparently - I learnt from another source). I can’t quite follow why this is, I would’ve thought that pressing the lever simply moved the fluid down the cable and it is the pistons moving in that relieves/responds to that pressure. Why isn’t that the case? It feels intuitive that you need the bladder to bulge to be able to press the lever in, but is the fluid that fills the bladder separate from the fluid that travels down the cable to the calliper? I think I’m right saying that the bladder expands into a hole that has an opening to the outside air, to allow for its expansion and contraction - but if that bladder growing and shrinking is allowing the lever piston to move up and down its cylinder, the fluid in that bladder must be separate to the fluid filling the cable. Thanks again.

The bladder/diaphragm (depends on the company as to what they call it) is closed off to the piston as soon as the piston moves a certain amount. So, there is no growing or shrinking while the brake is pulled. It is simply sealed within the resevoir. Once the lever is released and the piston moves back in and opens the port to the resevoir the hose will fill with what fluid the hose/piston interface may need due to the advancement of the caliper pistons. The amount of movement is very very little on any given pull. There is some valving at the lever that is more complicated than this lets on as well.

This will not likely fix your issue. You may be looking at a new caliper.

If you have a DOT system the fluid can actually pull moisture through the hose and add to the volume of the system. This causes 2 issues. 1. you cant push the pads back in far enough to get new pads in. 2. moisture in the fluid lowers the boiling point making the fluid less effective. It is best to bleed the system if you have this issue.

@@parktool it's a mineral oil system, so shouldn't absorb water in the same way (I think). But also if you were to only push on one piston, you'd expect it to retract and the other to extend (if there is too much oil), but it's not budging.

One thing you can do to break that free is to stack two rotors on one another and stick them in the slot of the caliper and then pull the lever to advance the piston that is free. The piston will contact the rotors and will not be able to advance any further. The pressure will then go to the other stuck piston and will push it out. You can then clean the piston and should be able to push it back in. Leaving the pad in on the side that is moving will help keep the piston from advancing too far as well.

@@parktool Thanks, I might try that with something other than a couple brake rotors (to not risk bending them). Any idea how far out the pistons can protrude from the caliper before they come out of the seals completely?

Fred J. I would use oil because it will aid with rehydrating the seals, oil keeps everything fresh.

@@sixstringedwithabowl YES That makes sense. Thank you.

A new caliper would be the guaranteed solution. Rebuilding the caliper if a kit is available is another solution.

Will do. Thank you!

Many brakes will say what they are on the lever in small print. Others you will need to go to the manufacturers website to find out.

No. It will be pushed into the system polluting your dot or mineral oil.

These can be difficult. Remove the pads. Pass the narrow end of a cone wrench through the entire brake. This way you can pull it back evenly into the body. Grab either side on the wrench and apply force. If it is crooked, apply more pressure slightly to the high side and go slowly. Good pushing to you.