A low brake pedal that has to be pumped repeatedly to bring a vehicle to a stop may be due to a low fluid level, drum brakes that need adjustment or air in the lines. If the pedal comes up and the brakes apply normally, you probably don't need a brake job. But the brakes may need to be adjusted, or the lines bled to remove air.
If the pedal feels "soft" or "spongy" instead of firm, there's probably air in the system. This will require "bleeding the brakes" to remove air from the lines, calipers and wheel cylinders.
The first thing that should be checked is the fluid level in the master cylinder reservoir. If the level is low, there's a leak somewhere in the hydraulic system that must be found and repaired. Adding fluid will only cure the symptom, not the cause, and sooner or later the level will be low again creating a dangerous situation. So look for leaks around the master cylinder, wheel cylinders, brake calipers, rubber brake hoses and steel brake lines.
If the fluid level is okay, the adjustment of the rear brakes should be checked next (assuming the vehicle has drum brakes in the rear -- if it has drums all the way around, check the front drums first, then the rears). The shoes should be close enough to the drums to produce just a hint of drag when the wheels are rotated by hand. An excess of slack probably means the self-adjusters are either frozen or fully extended.
If adjusting the drum brakes fails to eliminate the low pedal, the wheel and drum will have to be removed so the adjusters can be freed up or replaced, and/or so the worn brake shoes can be replaced.
If the vehicle has rear disc brakes, the adjusting mechanism in the rear caliper pistons that maintain the correct pad-to-rotor clearance may be corroded, frozen or worn out. In most cases, the piston assemblies cannot be rebuilt and must be replaced.
If the fluid reservoir is full and the brakes are properly adjusted but the pedal is low (or feels spongy), there is probably air in the brake lines. Air is compressible, so every time you step on the pedal, the bubbles collapse instead of transferring pressure to the brakes. The cure here is to bleed the brake lines following the factory recommended sequence.
Brakes are usually bled in a specified sequence (always refer to a shop manual for the exact procedure for your vehicle). Usually the rear brakes are bled first, then the ones up front on most rear-wheel drive cars and trucks. But on front-wheel drive cars and minivans, the hydraulic system is split diagonally so the brakes are bled in opposite pairs (right rear and left front, then left rear and right front). Following the proper sequence is important so air doesn't remain trapped in the lines.
On late model GM and Fords cars with quick take-up master cylinders, the quick take-up valve takes about 15 seconds to reseat after the brake pedal has been depressed. If the pedal is pumped too quickly while manually bleeding the system, you may never get the pedal to firm up. Most professionals use pressure bleeding equipment to bleed the brakes because it is faster and easier.