if you pump pedal , with engine off does pedal get harder, all 1991 c1500 did not have antilock brakes , if you give me vin number can look at options to see if built with antilock, with the power assist on this vehicle , booster will let you push pedal all the way to floor, if you drive vehicle, how far down does pedal go on normal stop, how about rear brakes are they out of adjustment, here is diagnosis chart.....
A brake pedal that sinks all the way to the floor is the result of the master-cylinder not developing any hydraulic pressure. This can be caused by a severe fluid leak which has emptied the master-cylinder, or by complete failure of the seals inside the master-cylinder.
A pedal that slowly sinks to the floor is the result of hydraulic pressure leaking off. This may be from an internal leak in the master-cylinder or an external brake fluid leak.
NOTE: Rear brakes which are extremely out of adjustment in conjunction with air in the hydraulic lines can also produce this symptom. Always verify that the shoes/linings are properly adjusted and the brakes have been bled prior to replacing a master-cylinder.
WHAT TO CHECK FOR
- Verify that the fluid reservoir is full.
- Verify that there are no signs of leakage at the area where the master-cylinder bolts onto the brake booster. If any leakage is indicated, the master-cylinder should be replaced.
NOTE: Minor leakage will often be indicated by "bubbled up" paint on the brake booster just below the master-cylinder. Brake fluid is very corrosive to painted surfaces.
- Inspect the rubber gasket on the inside of the master-cylinder lid. If the gasket is swollen or bloated, the brake fluid is contaminated with oil.
- Siphon or scoop a small amount of brake fluid out of the master-cylinder and place into a styrofoam cup filled 2/3 of the way with water. Brake fluid will mix with the water while oil contaminants will float on the surface and dissolve the styrofoam.
NOTE: If any oil contamination is present ALL brake components containing rubber seals will need to be replaced or overhauled. This includes the master-cylinder, both front calipers, both rear wheel cylinders, and all flexible brake hoses.
- Trace all brake lines from the master-cylinder to the calipers and wheel cylinders. Replace any lines that shows evidence of leakage.
- Remove all wheels and inspect the calipers and wheel cylinders for brake fluid leakage. Replace any component that shows evidence of leakage.
- If no signs of external leakage can be detected, bleed the brakes and verify the shoes/linings and emergency brake are properly adjusted.
- If the pedal still sinks to the floor, replace or rebuild the master-cylinder (Non-Antilock Brake Systems only, For ABS systems see Antilock Brake Systems/Testing and Inspection).