Did a totally routine weekend brake refresh that has now graduated into a part-time haunting. New front rotors and pads, cleaned and greased slide pins, flushed with fresh DOT 4, bled it like it owed me money. Now the pedal does a neat party trick: after a pothole, speed bump, or a long sweeping turn (especially right-handers), the first press sinks halfway like it’s on vacation, the second press is rock solid like nothing ever happened. No warning lights. No fluid loss. Straight-line cruising on smooth pavement? Perfect. Engine off, pedal is firm as the moral high ground. Booster works. Car stops straight. No weird noises. Just an occasional “first press is mush, second press is chef’s kiss.”
Vehicle setup for context only: front single-piston floating calipers, unitized front wheel bearings, 4-wheel ABS, conventional parking brake.
What I did and checked:
- Pressure bled, then did the ABS bleed routine with a scan tool, then bled again for good luck. No visible air.
- Rotors are new and clean, hats rust-free, lugs torqued in sequence.
- Pads fit properly, shims in, ears lubed, no binding.
- Slide pins move like they’re on roller skates.
- No seepage at hoses, calipers, master, or HCU. Fluid holds level.
- No play detected with the basic 12-and-6 shake test, but that test is about as precise as using vibes to set timing.
My running theory: pad knock-back thanks to hub/bearing play or rotor runout. The bumps or cornering nudge the rotor, pistons retract, and I get a longer first stroke. Second press brings the pads back to the rotor and life is good again. But before I go spelunking for a dial gauge or start throwing wheel bearings at it like confetti…
Questions for the hive mind:
1) Is this textbook pad knock-back? Any quick driveway proof tests without a dial indicator? For example:
- Clamp a zip tie to the knuckle as a pointer against the rotor face, spin, and watch for gap change?
- Mark and index the rotor to the hub, rotate 180 degrees, and see if the problem follows the rotor or stays with the hub?
2) Could trapped air in the ABS hydraulic unit behave like this only after bumps/turns? Or would that be spongy all the time, not just on the first press after a jolt?
3) Can an internally collapsed flex hose act like a one-way valve and cause first-press mush, second-press normal? Or does that usually cause dragging/pull instead?
4) Any chance low-drag calipers + a bit of rotor runout is all it takes here? If so, acceptable runout target to avoid knock-back on these setups?
5) DIY best practices to separate rotor runout from hub runout without a proper indicator:
- Measure hub face first with a makeshift pointer, then install rotor and compare?
- Use correction shims vs. just replacing the hub if hub face isn’t flat enough?
6) Unit bearing people: how sensitive is this to axle nut torque? Worth re-torquing to spec before I blame the bearing, or is that just placebo maintenance?
Bonus: rear drums/shoes (if applicable) being slightly out of adjustment-could that masquerade as first-stroke sink only after bumps/turns, or would it be consistent every time until reverse-apply/self-adjust?
I’m trying to avoid the time-honored tradition of replacing everything in a 10-foot radius. Convince me it’s not the front bearings so I can spend money on something equally unsatisfying.