Worn suspension bushings can also cause such symptoms. The normal loads on suspension parts lead to bushings getting an oval-shaped worn spot typically (where the bolt passes through); applying the brakes however changes the overall force direction/pattern. The wear in the bushings, when subjected to braking forces, may allow suspension parts to move in undesired ways. Putting the car on a lift and FIRMLY wiggling the joints is how most shops look for such problems. The same tests will also find worn ball joints and other worn suspension parts. Another issue I've seen on Hyundais is the metal sleeves inside the rubber bushings (that suspension bolts pass through) gets rusted. This rust jams the sleeve against the bolt, preventing the two parts from twisting/rotating around each other freely as the suspension moves up/down. This is quite common on the lower shock absorber mount. When this happens, the suspension has excessive friction... so more road irregularities and vibrations make it into the cabin. Under moderate to heavy braking, where nose-dive is common, the nose-dive is suspension movement that is over-resisted by this friction leading to odd feelings in the cabin. In extreme cases, the rust really jams the bushing to the bolt for nominal suspension motion but, when driving over a larger bump, the larger suspension motions generate enough force to override the jam - the bushing "pops" free often with an audible clunk and a "whack" to the cabin.
Another gotcha is over-tightening the lug nuts; this can bend/warp a brake rotor leading to pulsing in the brake pedal.
If the brake caliper does not slide properly, the brakes will not release properly. This creates extra drag while driving, and creates heat in the affected brake. Calipers are mounted via two bolts that pass through the caliper typically; when you apply the brakes the caliper piston pushes on one brake pad. Once that pad is against the rotor, further piston pressure causes the whole caliper to move until the other pad is against the rotor. At this point, the brakes are hardly doing anything... more pressure on the brake pedal though pushes the piston harder, making it press one pad more firmly into the rotor; the forces also make the caliper slide a tad bit more, squeezing the other pad into the rotor as well. When working properly, the sliding action of the caliper assures both pads are equally sharing the work. However, when the grease on the caliper bolts is missing or rust has built up on the bolts, the excess friction leads to lopsided pad pressures. And that same friction means the caliper won't slide back when you release the brakes so one brake pad never fully "lets go" and drags. Water in the brake fluid can lead to a thin layer of rust on the piston or inside the caliper; this rust creates friction on the piston so it doesn't release properly (when you lift off the brake pedal) leading to brake drag. Any brake drag creates heat which can warp the rotor. That same heat screws up the pads too leading to inconsistent braking action.
Contaminates on the rotor (especially grease, oil, or leaking brake fluid) lead to spots where the pad-to-rotor grip drops; that induces a pulsing feel in the braking action but not in the brake pedal as it doesn't cause fluctuations in brake fluid pressures. It feels similar to trying to brake on patchy snow or patchy ice. Cleaning the rotors (after the car has been parked and the brakes have cooled!) with a can of spray brake cleaner fixes this. Spray both sides of the rotor, rotate the
wheel a bit to access the part of the rotor hidden by the brake pads, and spray again.
A normal brake pad replacement job should include removing the rotors and having them "turned" on a brake lathe machine. This machine cuts a thin layer of metal off both faces of the rotor so any grooves or other damage from the old pads is removed. It also makes the rotor perfectly flat or "true" - i.e. it cures any warpage of the rotor - which eliminates the usual cause of brake pedal pulsing. Rotors can typically be turned once or twice before they are too thin; once too thin they must be replaced.
The ABS HECU issue that lead to the recall was due to corrosion building up on the HECU internal parts, jamming them. The parts are coated with a material that lets them be machined very smooth and to very tight tolerances. But this coating wasn't too resistant to water... When jammed, the net result was NO braking action - the HECU typically got stuck in the position that relieves brake fluid pressure to un-lock a locked (skidding)
wheel. The brake pedal, in this case, would often go to the floor with little to no braking action. Standard DOT-3 brake fluid is hydroscopic which means it absorbs humidity/moisture from the air. The DOT-4 fluid called for by the recall is much less hydroscopic. Later HECUs were built with internal coatings that were more resistant to water trapped in the brake fluid. At least that's my understanding of the situation.
mike c.
