aaronmichael

Hello all,


I'm usually on here asking about wheel building questions but I have a hydraulic disc brake question this time. I understand the basics of hydraulic disc brakes but not completely and there are still a few things that perplex me. This is one of them.


I had a friend bring me their bike with very worn down brake pads on their rear Tektro Auriga disc brake. It was almost down to the backing plate. Therefore the pistons were sticking out pretty far. When I tried to push the pistons back into the caliper to install some new brake pads, they would not want to stay recessed in the caliper. While putting pressure on the pistons, I noticed some fluid started to leak out of the backside of the lever where it pushes the master cylinder piston (near the lever pivot). I immediately stopped what I was doing and unscrewed the bleed port on the lever a turn. With the lever bleed port slightly open, I pushed the pistons back in and some fluid started to come out. I was finally able to get the pistons to stay retracted, got a new set of pads installed, cleaned the lever, and everything felt fine. However, when the rider took it home he said it continued to leak out of the backside of the lever and when he took it on a ride shortly after, it kept leaking. Even after that excess fluid leaked out, the brake still feels pretty good.


Looking for some information on what I did wrong here. Did I possibly damage one of the seals by putting too much force on the pistons pushing them back in? How is there "too much fluid" in the system? Why wouldn't the "excess" fluid in the caliper travel and stay back up in the lever? Any information would be greatly appreciated. I would love to understand more in depth how this all works. Thank you!

cpach

Not your bad. Sometimes people will bleed just the lever with the pads partially worn and installed, which will slightly overfill the system with new pads. Still, it's not normal for the reservoir or master cylinder to fail trying to press in the pistons on a slightly overfilled system. The lever was damaged before you touched anything. I haven't seen this on auriga before (and I see them pretty often), but it is a fairly inexpensive brake. If it's less than 2 years old Tektro would probably accept this as a warranty if processed through a bike shop.

aaronmichael

Thank you for the response. I mean, I was pushing pretty hard on the pistons to try to get them to stay back in. There were some significant scuffs on the brake lever. Could it have been damaged in a crash? The guy did say that it was never leaking before he brought it to me for the repair. Does the master cylinder piston only push a partial amount of fluid that's in the lever? Is there another chamber in there that holds some "spare" fluid? I'm not so concerned about the cost of fixing the problem, I'm more so trying to understand how the system works and why the fluid wouldn't go back into the lever when pushing the pistons back in. Would this issue ONLY happen if someone bled the brakes while the pistons were pushed out? Any additional information or links to articles would be appreciated, thanks!

cpach

Again, what you did sounds reasonably normal. I guess if you used particularly crazy force maybe? I do immediately open the bleed port if I feel like the system is overfull.

Basically, there is a reservoir of fluid in the lever that has a small port going into the bore the master cylinder travels. When the master cylinder is at rest (with the lever not depressed) fluid freely travels from the reservoir to the rest of the brake system. This is necessary because the pistons will advance on the quad seals to maintain a consistent distance to the rotor as the pads and rotor wear, causing more fluid to be stored in the caliper. When the lever is depressed, the master cylinder moves past the port to the reservoir and the system is closed and compressed. The master cylinder seals are designed for pretty high pressure, although some systems do poorly when they're subjected to considerable force bleeding the lever end with suction. If you went really crazy you may have been able to cause leaking from the reservoir, which is really only sorta-closed. They tend to be part of a single piece of metal abutting the master cylinder bore with a rubber gasket sealing the cap at the top of the lever. If anything failed, it's this, and probably not catastrophically. If anything, overpressuring the reservoir might have caused the gasket to leak a little, and there's some fluid that is still soaked into the lever pivots etc.

If its leaking from the master cylinder it's pretty much definitely not your fault, as that takes the entire force of the braking system when used.

aaronmichael

Right on,

Well thank you for all of the information. I'll pass along the information and will probably end up replacing the lever anyway. Take care!