For vehicles with rear drum brakes
If your brake pedal sinks to the floor while pressing on your brakes, you probably have a leak in the master cylinder. The master cylinder in your vehicle has two pistons, a primary piston and a secondary piston, each of these pistons operate the front or rear brakes.
These pistons are sealed using a cup and it’s most likely that the primary cup is leaking brake fluid back behind the primary piston. This would account for the brake pedal sinking as you press on it to stop your vehicle.
If a leaking piston is why your brake pedal sinks to the floor, you would have to replace the master cylinder with a new rebuilt cylinder.
If your brake pedal doesn’t go all the way to the floor, but still goes down much further than it used to, then the first thing you need to do is check the brake fluid level.
To do this, open the hood and locate the brake fluid reservoir. The brake fluid reservoir usually sits on top of the master cylinder.
While looking at the reservoir rock the vehicle back and forth in order to see where the fluid level is. There is no need to remove the reservoir cap while doing this.
Is the brake fluid level low?
If your answer to this is yes, then your problem is a lack of brake fluid.
If your brake fluid is not low, however, then the problem is likely being caused due to your vehicle’s rear drum brakes.
This indicates that your brake shoe linings are more than likely worn.
In vehicles with rear drum brakes, you will have to press further down on the brake pedal as the shoe linings become more worn over time.
You can fix this by installing new rear brake pads.
For vehicles with all disc brakes
If your brake pedal sinks to the floor and you have all-disc brakes there are only three possible causes.
1. Your brake fluid is low. (you can check this in the same way as explained above)
2. You recently had work done on your brakes and the pushrod on your master cylinder was not adjusted properly.
3. You have an internal leak in the master cylinder.
Unlike jeeps that have rear drum brakes, the wearing of the shoe linings on vehicles with all disk brakes will not cause the brake pedal to sink to the floor.