3-second answer: The proportioning valve (under the master cylinder) has a shuttle that slides off-center when a brake line loses pressure — it blocks the circuit and you can't get a good pedal. Reset: with the system full and bled per side, the shuttle self-centers when pressure balances — or depress the reset button/pin on the valve (center it by hand, then bleed each side again).
Why it's the hidden villain
- When you open a brake line (caliper, hose, line), the shuttle slides toward the low-pressure side and seals it off — the pedal goes soft/floor-ish even after bleeding.
- This is the classic 'I bled the whole system and still have no pedal' story on OBS trucks.
The correct bleed (with the valve)
- Fill the master, bleed the master (bench or on-car with the lines loose).
- Bleed in order: right rear, left rear, right front, left front.
- If the pedal is still soft after the first pass: open a rear bleeder, have a helper push the pedal HARD (this pushes the shuttle back to center), close, repeat on the other side.
- Some valves have a physical reset — depress it while bleeding.
- Road-test and re-bleed if the brake light (RABS) or pedal feel is off.
When it's NOT the valve
- Pedal slowly sinks with pressure held = internal master leak (see brakes).
- Spongy but consistent = air (keep bleeding).
- Low/hard pedal with no travel = the pushrod or a collapsed hose.
community-standard · verify
Threads
- FTE search: proportioning · brake pedal