I have a 2006 PT Cruiser. Sometimes when I am at freeway speed I have had to hit my brakes in traffic. When I apply them they pedal seems hard and stiff but the car does not slow down so I bear down on it more and eventually the hydraulics kick in and slow the car down. This has happened twice on the freeway. The other day I was backing out of my driveway and I hit the brakes and it did the same thing it kept rolling. I pulled it forward and hit the brakes and it stopped. I did not have anymore problems the rest of the day. I pulled the wheels today and I see no problems with brakes linkage or see any evidence of leaks. What could the problem be. It only happens periodiocally.

 

The master cylinder provides hydraulic fluid pressure to cause the slave cylinders to engage the friction component (either shoes or pads, depending on the car's configuration) to engage the drums or rotors which slow the rotation of the wheel. The master cylinder is mounted to the firewall underneath the car's hood on the driver's side. Check the following to determine if you have a problem with the master cylinder:
Check the fluid level in the brake fluid reservoir. Some cars have a translucent resevoir that allows you to see the fluid through the side, others require you to unscrew or remove the clamps and take the lid off the reservoir. There should be a fill line and add line marked clearing on the side of the reservoir.
Check the area around the master cylinder for leaking brake fluid. This can indicate a damaged reservoir or the seals inside the master cylinder itself leaking and allowing the fluid to escape.
Slave cylinders cause the brake shoes or pads to engage the drums or rotors. They are located on each axle, either on the rotor assembly or inside the brake drum, depending on which system your car utilizes.
Check each hub for leaks. You can usually do this by looking at the back of each wheel to see if there is evidence of fluid dripping or accumulating on the inside surface of the wheel assembly.

 

You could replace your break pads and bleed the brake fluid and replace it with fresh fluid. If you have to press harder than normal to stop then ask your mechanic to check your power boost unit. Hope something I said helps! Good luck