sw20

Hi All,

One side of my front disc brake piston doesn't fully retract, so that my brake is rubbing slightly. I made the mistake of pulling the brake lever whilst the front wheel was off when i was transporting the bike.

I have tried taking the pads out and pushing the piston back but it doesn't go back fully, and it takes quite a bit of pressure to do this.

Is there an easy fox for this?

Thanks

jimc101

Hydraulic discs are pretty easy to service, plenty of videos on youtube for this.

Without knowing the specific brakes you have, you could undo the top cap at the lever end (the reservoir), and then use a hex key (probably 8 or 10mm) to push the piston back in with the pads removed, this will remove the pressure issue, and you should be able to see fluid being returned to the reservoir when your doing this.

fietsbob

maybe some dirt had gotten in the slave piston seal?

corrado33

The pad shouldn't "fully" retract. It only retracts half a mm or so when you let go of the lever until it doesn't touch the rotor.

As for it not retracting fully when pushing on it. Make sure you push on it evenly, not cockeyed. And push harder (with a plastic tool, tire levers generally work well.)

Also, your alignment may be off. Look up videos for how to align your caliper.

trailflow1

If they still wont retract after removing some fluid (i would try this first). Try spraying some silicone lubricant spray around the pistons. It is safe to use and wont damage the rubber seals. Do not use any other types of lubricants because they will damage the seals.

sw20

Thanks for all the replies!

sorry for the noob question, but will I need to top up the mineral oil if i do this?

Thanks

corrado33

Theoretically, no. He's thinking that the air pressure in the sealed system is making it hard to push the piston back, so by taking the cap of the reservoir off, you're removing the pressure. Make sure you rotate the reservoir so it's level (and no fluid will spill out) before you do it.

Also, not all hydraulic brake systems take mineral oil. Some take brake fluid (Dot3 or 4). Make sure you know what kind your brakes take.

sw20

Thanks corrado, I've got Tektro brakes which I believe are mineral oil.

Maybe I'll get some, just in case!

fietsbob

Without knowing the brake, no one can say whether it uses DOT brake fluid, or mineral oil.

Have you read the manual yet?

grizzly59

If any brake system is made for DOT brake fluid, adding mineral oil will most likely ruin every rubber part inside the system. It will never work correctly without a surgical cleaning, and replacement of all the rubber parts.

Scoot66

try DOT5:1, not 3-4