why does my brake pedal go to the floor

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SYMPTOM SUMMARY The brake pedal goes to the floor when pressed. There may be little or no braking action when the pedal is depressed. The Red Brake Warning Lamp may be illuminated indicating a system failure. USUAL CAUSE Very low brake fluid or a defective Master Cylinder is the usual cause of this symptom. The master cylinder pressurizes the brake system when the pedal is depressed and provides hydraulic fluid to each of the wheels to apply the brakes. The braking system is designed to illuminate the Red Brake Warning lamp when a hydraulic failure is present. DIAGNOSIS If you have just performed other brake work on the vehicle and the hydraulic system was opened, the most probable cause is air in the system. The system will need to be bled according to manufacturers recomendations. The first step in diagnosing this symptom is to inspect the brake fluid level at the master cylinder. If the fluid level is low, the system will need to be inspected for leaks.

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The calipers, wheel cylinders (on drum brakes) will need to be inspected. Brake pads that are completely worn can cause low fluid level. This is a result of the caliper pistons being completely extended, causing most of the brake fluid to reside in the calipers. The brake pads and shoes (if equipped) should be inspected for signs of brake fluid contamination indicating a leak. Inspect the calipers for leaks at the rubber dust boot around the pistons. The dust boot on the wheel cylinder will have to be pulled off slightly to inspect for leaks. Check the hydraulic lines, proportioning valve and flexible hoses for leaks. Inspect the master cylinder where it mounts on the brake booster for signs of leaks. Loosening the master cylinder retaining bolts and pulling it back slightly, will usually result in detection of a leak if present. If leaking, the fluid will run out of the mating surfaces. In this case, the master cylinder will require replacement.

If there are no fluid leaks present or the fluid level is normal and the brake pads and shoes (if equipped) are in good condition, the most probable cause of the symptom is a defective master cylinder. Determining the cause of the failure will dictate what corrective action is necessary. If the hydraulic system needs to be serviced, as with replacement of the master cylinder, it will need to be bled of all the air once it has been resealed. Refer to the manufacturers specific bleeding procedures. PRECAUTIONS, TIPS, and NOTES
If the vehicle is equipped with ABS, certain ABS components can cause this symptom. In this case, the system should be serviced according to manufacturers recommendations. If servicing the hydraulic system, use caution not to contaminate the system with dirt, debris or water. Always use the recommended brake fluid DOT rating. This information can be obtained from your owners manual or off the cap on the master cylinder. (Lance owned his own auto repair shop for 30 years before retiring in 2006. ) One of the valves in the brake servo (booster) is stuck open.

When you start the engine the vacuum produced by the engine (or a separate vacuum pump) is used to assist braking pressure from your foot. As you press the brake pedal, a valve in the servo opens to allow the vacuum to pull against the diaphragm in the servo to assist you. If a valve is stuck open it will keep sucking until the pedal goes to the floor and will only release once the engine has stopped. Presumably it is putting the brakes on when this happens. If this is true, it is nothing to do with bleeding. Depending on the design of the servo, there may be a pipe that connects the servo casing to the servo valve block, if this is leaking or missing, the same symptoms would occur. Any air leak into the casing in the rear of the servo body will also produce these symptoms.

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