Question: My brake pedal slowly sinks to the floor when
I hold my foot on it. What's wrong?
Answer: You either have a fluid leak in your brake system
or your master cylinder is defective. Either way, your brakes
need immediate attention.
If the brake warning light is on, you most likely have a fluid
leak. Your vehicle may not be safe to drive in this condition!
You should have the brakes inspected as soon as possible to determine
where the fluid is leaking (usually a hose, brake line, brake
caliper or wheel cylinder) so the necessary repairs can be made.
If the brake warning light is not on, it does not necessarily
mean you do not have a leak. The warning light only comes on when
there's been enough fluid loss to create a pressure differential
between the two sides of the hydraulic system that actually apply
the brakes.
The brake system is divided into two hydraulic circuits. On most
rear-wheel drive vehicles, it is divided so one circuit applies
the front brakes and the other applies the rear brakes. On front-wheel
drive cars and minivans, the system is usually split diagonally.
One circuit works the right front and left rear brake, and the
other works the left front and right rear brake. This is done
for safety purposes so if one circuit loses all its brake fluid
and fails, the vehicle will still have one remaining circuit to
apply two wheel brakes.
A quick way to check for leaks in either circuit is to simply
check the fluid level in the master cylinder reservoir. The reservoir
is divided into two chambers (one for each brake circuit). If
one chamber is unusually low or empty, there's a leak somewhere
in that circuit. The brakes should then be inspected to check
for fluid leaks. Wet spots around hose or line connections, or
fluid leaking from a disc brake caliper or drum wheel cylinder
would indicate a serious problem that needs immediate attention.
If the brake warning light is not on and there are no apparent
leaks, then the master cylinder may be worn or leaking internally
allowing the pedal to slowly sink when pressure is applied to
it. This type of condition will be most noticeable when holding
constant pressure against the brake pedal at a stop light. If
the pedal sinks or requires pumping to keep the car from creeping
ahead, the master cylinder needs to be replaced.
On some vehicles with rear-wheel antilock brake systems (ABS),
it's also possible that a leak in the ABS unit may cause a similar
sinking pedal condition.
