Wooshing sound when you step on brakes
 

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?

Yeah I'd put money on it being the brake booster, they fail in several different ways but all related to the vacuum diaphram leaking in some way or another.

Conanski- my understanding was that usually when a booster fails the result is a very stiff pedal that takes a ton of effort to push, not a spongy/soft one? Is that not the case?

OP- does it hold a steady amount of pressure when you're on the brake, or does the pedal sink? Also check for vacuum leaks, and make sure the booster is getting plenty of vacuum.

I'd start by checking the entire system for leaks, and if none are found it could be the master cylinder is bad. Of course if you replace the MC thats a good time to replace the booster too. That said, the fact that you're hearing an air "whoosh" when you apply brake could support the booster theory.

I'm sure someone with more knowledge will chime in, I could be wrong.

A quick whoosh when you step on the pedal ain't nothing to fret over. Now if it makes a continuous noise while you hold it down, then yes your brake booster is failing. It won't be hard to push the pedal til the diaphragm blows out, in my experience.

BUT: that shouldn't give you a soft pedal. When you're stopped and put pressure on the pedal, does it slowly sink toward the floorboards or stay put?

I assume you already looked around for leaks, but have you checked the master cylinder itself? Usually what kills a booster is the master cylinder blows a seal (on the pushrod end, the end of it facing the driver) and brake fluid leaks into the booster. The fluid eats the seals in there, and then you get a leaky booster.

Might wanna loosen the MC up and take a look... but usually you can see the crap running down the front of it. Either way, if you replace the booster then get a new MC or at least inspect your current one so that it don't trash your new booster.

I have a new brake booster and no vacuum leaks so my system is working great and each time I press the brakes I hear what I would also call a 'whoosh' sound. I think this is normal operation of a brake booster, at least it seems to be for the one I have. So I would think just hearing this sound doesn't mean you have something wrong.

Have you checked the front brakes to see how they are?

You say you have a soft pedal when you step on the brakes, that can have a lot of interpretations. I assume the pedal goes almost to the floor before the brakes work? What happens when you pump the brake pedal several times? Does the red brake light in the dash ever come on when you hit the brake pedal? Does the red brake light come on when the key is in the start position, so you know it is working? A good way to check that is don't press the clutch down or with an automatic transmission put in reverse or drive and turn the key to start.

vacuum booster test
 

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.

Note: 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.

Note : 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

are you used to driving a different vehicle? i have found that these trucks have a softer brake pedal than most other cars , even if they are in good working order. mine also makes a very short woosh noise when i step on the brake pedal, and mine stops just fine, just my 2 cents

X2, that is the way they fail.
Or at least the bad ones I have had did.