* Residual Pressure Valves: These valves are designed to retain a small amount of pressure in the wheel cylinders to prevent the brakes from dragging. However, a malfunctioning or incorrectly adjusted residual pressure valve could build up excessive pressure over time, leading to a stiff pedal. This is a prime suspect given the symptom appears *after* driving for a while.
* Proportioning Valve (if equipped): Some vehicles of that era had proportioning valves to adjust braking force between the front and rear. A stuck or faulty proportioning valve could cause pressure buildup in one circuit, leading to pedal stiffness.
* Power Booster (if equipped): Even if a Falcon from '62 wasn't standard with power brakes, some owners added them. If it *does* have a power booster, it could be failing internally, losing its ability to assist with braking pressure after some heat buildup. A vacuum leak in the booster system would also cause this.
* Brake Lines (hard lines): While you replaced the flex lines, the hard brake lines within the chassis could be corroded internally or have a kink restricting flow. Rust particles or debris could be getting dislodged and clogging things as the system heats up.
* Master Cylinder Push Rod Adjustment: The pushrod from the pedal to the master cylinder might be incorrectly adjusted, causing the master cylinder to not fully return after brake application. This leads to pressure buildup in the system.
* Caliper issues (if equipped): While less likely on a 1962 Falcon, it is worth considering, particularly if the vehicle has undergone some modification. Sticking calipers could hold pressure after use.
* Wheel Cylinder Installation/Seals: While you replaced them, improper installation or a defective seal in a new wheel cylinder could cause a piston to stick slightly, leading to a gradual pressure buildup.
Troubleshooting Steps:
1. Check for Leaks: Carefully inspect the entire brake system, including hard lines, for any leaks. Even a tiny leak can cause problems over time.
2. Inspect Residual Pressure Valves: Verify that these valves are operating correctly and are properly adjusted.
3. Bleed the Brakes Again: Thorough bleeding is crucial after a brake overhaul. Make sure all air is removed from the system.
4. Examine the Master Cylinder Pushrod: Ensure it's correctly adjusted and moving freely.
5. Check the Proportioning Valve (if equipped): Inspect for binding or other problems.
6. Test the Power Booster (if equipped): Verify proper vacuum assist. Listen for leaks.
7. Examine the Hard Brake Lines: If possible, have them pressure tested for leaks or obstructions.
The fact that the problem develops *after* driving suggests a pressure-related issue, rather than a simple leak. Focus your troubleshooting on the components dealing with pressure regulation and flow. If you're not comfortable working on brakes, have a qualified mechanic diagnose the problem. Brake failure is extremely dangerous, so don't take any chances.
