Know the feeling? You're driving along comfortably, freewheeling without braking and the car comes to a stop with a slight jerk. I used to have a car that did this, when it did, I knew straight away, here we go again.
If we can trace the problem without a doubt, to a defect in the (hydraulic) brakes, e.g., by jacking up the car and turning the wheels, then there can be three possible groups of causes: Either the vehicle has a construction error or it is somewhat older or something has been mounted or adjusted wrongly.
Let's begin with a construction error. We'll pick out one of several possibilities, e.g., the tilting of the piston in the brake caliper. Basically, what is responsible for the releasing of the brakes, are small springs (drum-brakes) or in disc-brakes, rubber rings or disc-impact. In any event, the forces which cause the brakes to release are often much lower than the force applied when braking.
Thus, a tilted, or for any other reason, sluggish piston, can not be brought into position correctly by the enormous pressure applied when braking, it jams at a certain point causing not only one-sided wear and tear on the particular brake, but through friction while driving it can heat the brake to glowing temperature.
This, by the way, is valid for almost all errors which prevent the brakes from releasing properly. The second group of causes has to do with the rubber which swells and is found, e.g., in old brake lines which swell inwards. The brake pressure still comes through but it's return is obstructed.
While one can hope (mostly in vain), to solve the problem of tilting pistons by polishing, as far as swollen tubes are concerned, the only way out is to replace them. Of course, deposits in the hydraulic system can also gather so skillfully, that a blockage can also occur.
The third group of errors concerns the probably, most important component in the hydraulic system, the master brake cylinder. It is always active, not only when braking. The problem here can lie in blocked drillings to the reservoir. That, or the pistons are in the permanently applied state.
After one has examined the problem from the brake pedal aspect, all one can then do, is to syphon off the brake fluid and dismantle the reservoir. With a bit of luck, depending on the type and the position, one can look through the respective drillings into the cylinder.
If the brake pedal is not pressed and one can see the gaiter or even part of the piston, it's possible that the problem has already been found. The piston rod then has, in any event, too little ventilation play. The reason for this can be a misadjusted pedal, incorrect assembly or a piston which is not returning properly.
Of course, the brake booster could also be to blame. Should it come into action without the brakes being applied, the brake can also not return to the released condition. It may help, to simply disconnect it, and to seal the opening to the engine, then, during a test drive, to pay close attention to the drastically increased pedal forces. 10/11