Most vehicles on the road today rely on a hydraulically operated braking system, which is generally acclaimed to be the most efficient option available. This system will work for months or years at a time without question so long as the owner performs regular maintenance, but sometimes it will require some extra attention. If you've recently found your braking system to be inconsistent, as one minute it will fail to slow down the vehicle but the next it will simply "grab," what could be going wrong?
As you may know, most cars today have a disc brake system, which is made up of a rotary disc, a caliper and renewable brake pads. When the driver puts pressure on the pedal, hydraulic fluid forces the pads against the disc, and this will decelerate the wheel. Within each caliper, you will find a piston that is meant to push the pads forward and then release at the appropriate time so they can return to their seated position. Sometimes, these pistons will need to be replaced or reset, and given the type of problem being reported here, this would appear to be the case.
In order to assess the condition of the piston, the caliper will have to be removed. The process will depend on the make and model of the vehicle, but care should be taken to remove the caliper properly and without damaging the hydraulic brake line.
Once the caliper has been removed from the wheel assembly, the brake line should be sealed and tied out of the way until everything needs to be reassembled. The caliper can then be inspected to look at the condition of the piston and the accompanying seals.
It may be possible to determine the condition of the piston by looking at it in place, but often, it will have to be removed. This will require a compressed air gun or similar, as pressure will need to be applied to the brake line hole in order to eject the piston. Care should be taken to ensure that this is done properly and without any risk of injury.
When the piston is free of the caliper, it should be cleaned carefully to assess its proper condition. It may have been subject to wear and tear and you may see evidence of pitting or rust. In this case, the piston will need to be replaced.
How to Proceed
This job should be performed on both of the front wheels, and potentially the rear axle as well. However, it is not possible to do this at home unless you have the correct tools, so in most cases, the vehicle should be taken into a brake repair shop instead.
I've been running an auto service shop for the past 30 years. The trends in cars have changed a lot over that time, and cars are becoming a lot more complicated and automated. People can't fix many items on their own cars any more as you need a computer to diagnose most issues. People use their cars a lot more these days as we live further away from workplaces and schools so it's important for me to be able to get cars back quickly. I have had to keep learning about how the new cars run and I keep a lot of hints on this site so you can all learn the things you can do to make your cars run better.