Question: My brake pedal is low when I step on it, but it
comes up when I pump the brakes. Do I need new brakes?
Answer: 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. It usually
has nothing to do with the condition of the brakes and certainly
isn't grounds for a brake job.
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 check 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 rear). 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 Ford 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.
