Update: 1 year later
The levers got slow again. Not as stiff as before, just slow to return. On disassembly, the problem wasn’t the pistons, but the seals, which had swelled. Maybe because I put a drop of oil on them when I reassembled them, and oil can have seal swelling additives.
Anyway, all I needed was new seals. But new seals alone are not available, as far as I can tell. You have to buy an entire brake lever rebuild kit! However, I did find replacement pistons that come with new seals, and the pistons are machined aluminum, and they only cost $5-$10 each. Better than OEM quality, and perfect fit.
The brake levers on my MTB have been gradually getting stiffer to operate, more friction in the brake pull with a weaker return upon release. I bought this bike in late 2014 and have bled the brakes and replaced the pads. The lever stiffness has been gradually increasing. On my most recent ride on Tiger Mtn, the brakes were dragging pretty hard because the levers wouldn’t return. This was an incredible PITA on the steep uphills, and risks overheating the brake pads & rotors.
At Tiger summit, one of the other riders mentioned this was a known problem with SRAM hydraulic brake levers. When I got home I checked it out and found that was indeed true. Some people had returned their levers to have SRAM replace under warranty. But they said it was a PITA and took a long time because SRAM support dragged their heels not wanting to admit there was a problem. So I figured it was worth at least trying to fix it myself.
There are several YouTube videos about this. Here is one I found useful: https://www.youtube.com/watch?v=Ex882BIH-Fo
Here’s a summary of the problem and fix. Each brake lever has a small master cylinder inside, a piston with rubber seals. The piston is made of plastic and the cylinder is metal. Inside the master cylinder there is also a spring that pushes back against the piston to help it return to the neutral position. When the entire assembly gets warm/hot, the piston expands more than the cylinder, scuffing against the inside of the cylinder, increasing friction and getting stuck. It gets stuck so hard that the spring can’t push it back.
The solution is to remove the master cylinder piston and use fine (600#) emery paper to scrub off edge material (gently, smoothly, evenly), making it slightly smaller in diameter. To do this you must remove the brake lever from the hose, drain the brake fluid from the lever, disassemble the lever, remove the piston and its rubber seals, sand it down until it freely slides back & forth in the cylinder, clean everything up, reassemble it, then re-bleed the brakes. The procedure is tedious manipulating some tiny parts, and requires an experienced touch sanding down the pistons. But it doesn’t require any special tools, just the usual stuff: torx wrenches, brake bleed syringes, fresh DOT 4 or 5.1 fluid, etc.
The procedure was successful; my brakes are like new again. This took me almost all day, but I hadn’t done it before. I could do it again in less than half a day.
The problem is definitely not about the piston’s rubber seals. I removed those before sanding it, and the piston was super-tight in the cylinder even with the rubber seals removed and the cylinder cleaned. I sanded the piston until it was loose in the cylinder, easily sliding back & forth from gravity just tilting the assembly up and down.
The piston’s rubber seals are tight and one-directionally facing. Remove with care, ensuring you don’t scratch or score them, and ensure they’re facing the right direction when you reinstall. Before reassembling, make sure everything is scrupulously clean. You don’t want sanding dust from the piston or other crud inside your brakes!
I can’t figure out how or why this problem took 4-5 years to manifest. The piston was not deformed in any obviously visible way. Why didn’t this happen during the first year of ownership?