Originally Posted by reno138
1987 ford f-150 4x4 351 motor.
I have a soft pedal when I step on the brakes. I have ajusted the rear shoes. So that they have a slight drag. But still have a soft pedal. When I first step on the pedal I hear a quick Whoosh. Does this mean the vaccum booster is bad. Or how would I check it?
Power brake booster (vacuum operated)
(boring wish I had a scanner)
Begin the power booster check by depressing the brake pedal several times with the engine off to deplete any vacuum remaining in the booster.
Now, depress the pedal and start the engine. If the pedal goes down slightly operation is normal. Release the brake pedal and let the engine run for a couple of minutes.
Turn off the engine and depress the brake pedal several times slowly. If the pedal goes down farther the first time but gradually rises after the second or third depression, the booster is airtight.
Start the engine and depress the brake pedal, then stop the engine with the pedal still depressed. If there is no change in the reserve distance (the distance between the pedal and the floor) after holding the pedal for about 30-seconds, the booster is airtight.
If the pedal is "hard" when the engine is running (the booster isn't operating properly)
IF a defective booster is not diagnosed from those checks, inspect the check valve.
To do this, disconnect the vacuum hose where it connects to the metal pipe or the intake manifold (don't disconnect it at the booster). Apply pressure and suction to the end of the hose, making sure air only flows away from the booster. If it flows in both directions or if there is no airflow at all, replace the check valve. On some vehicles the check valve is located inside the hose, requiring replacement of the hose.
A restricted exhaust system could also be the cause of low vacuum.
some engines are fitted with vacuum pumps which help generate vacuum to power the brake booster and other accessories. A defective pump will most often make a loud rapping sound that rises and falls with engine speed.
You can also test the operation of the operation of the vacuum pump as outlined in the engine vacuum check in the next paragraph.
(Connect a vacuum gauge to the booster hose) Place the shifter in park if the transmission is an automatic) block the wheels and start the engine. Allow the engine to reach normal operating temperature, then look at the gauge - there should be at least 15 in-Hg indicated at idle. If not diagnose and repair the cause of low vacuum before condemning the power booster.
Engines with high performance camshafts will have a lower and somewhat erratic reading at idle.
here is just a listing of possible causes for the brake pedal "spongy" when depressed:
1.Air in hydraulic system.
2. brake shoes not centered in drum
3. brake drums machined to thin or excessively worn
4. cracked brake drum
5. brake shoes distorted
6. caliper or caliper mount flexing
7. master cylinder or power booster loose
8. brake fluid overheated (beginning to boil)
9. brake fluid contaminated
10. deteriorated brake hoses (ballooning under pressure)
11. soft of swollen caliper seals
12. defective residual check valve
13. broken brake pedal pivot bushing or bracket
and solutions (in order)
1 bleed the system inspect system for a leak
2 inspect drum brakes mount shoes correctly
3 inspect drums replace if diameter exceeds maximum allowable diameter
4 carefully inspect drums replace if necessary
5 replace brake shoes
6 inspect calipers and mounts for loose fasteners, cracks and other signs of fatigue. replace as necessary
7 tighten fasteners
8 bleed the system (temporary fix) replace the brake fluid (proper fix)
9 replace brake fluid
10 inspect hoses replace as necessary (it's a good idea to replace all of them if one hose shows signs of deterioration)
11 replace seals (if seals are swollen due to contamination flush entire system and replace all rubber components)
12 replace valve, bleed system
13 replace pivot bushing or repair bracket