Woosh sound

[gsb77]

When i push the pedal down there is a woosh sound. The truck will stop but the brakes are NOT as firm as before. The pedal will go all the way to the floor. As soon as i noticed this, i checked fluid and did have to add some. Have not noticed any leaks on calipers or drums. Anyone have any suggestions?

 

[Georgeb]

How much did you have to add?

 

[firebane]

Brake booster is bad.

 

[kdo58]

brake booster is bad, and i was loosing brake fluid, and could not find a leak, ended up the master cylinder was leaking into the brake booster.

 

[gsb77]

How big of a job is it to replace?

 

[firebane]

How big of a job is it to replace?

Its not a hard job or big really. The hardest part is crawling under the dash to undo the linkage to the break pedal.

 

[smoothandlow84]

I was gonna say it was the booster as well...but too slow to post a reply. The brake system isnt too difficult to remove and replace for a diy mechanic.

 

[chengny]

I am going to have go with a failing master cylinder because of this:

The pedal will go all the way to the floor.


Even without power assist (i.e. with the engine turned off), the degree of brake pedal travel should not be effected. The vacuum booster only serves to reduce the amount of force required by the driver to apply the brakes - it does not limit/reduce push rod or master cylinder piston stroke.

If the master's seals/cups are failing - and the brakes are applied - the result will be insufficient pressure developed at the master's outlet ports and in the external hydraulic circuits. Additionally, the piston will offer reduced resistance to the push rod/brake pedal.

Without normal pressure/resistance, the piston can more easily be forced into the master's bore. If the internal seals are worn bad enough, the piston can go right to the end of the bore.

A vacuum booster is just a sheet metal shell that contains two chambers which are separated by a flexible diaphragm - which offers very little resistance to application of the brake pedal.

There is a push rod that goes from the brake pedal arm, enters through the back side of the booster housing. Inside the housing it passes through the diaphragm and operates the control valve. It then exits at the front of the housing and extends into the master cylinder.

Bottom line is: if you have excessive pedal travel, it is because you are not developing design hydraulic pressure in the braking system. This could be caused by a failed MC, leaks in the external circuits or air in the system - but not by a malfunctioning vacuum booster.

 

[Georgeb]

I am going to have go with a failing master cylinder because of this:

The pedal will go all the way to the floor.


Even without power assist (i.e. with the engine turned off), the degree of brake pedal travel should not be effected. The vacuum booster only serves to reduce the amount of force required by the driver to apply the brakes - it does not limit/reduce push rod or master cylinder piston stroke.

If the master's seals/cups are failing - and the brakes are applied - the result will be insufficient pressure developed at the master's outlet ports and in the external hydraulic circuits. Additionally, the piston will offer reduced resistance to the push rod/brake pedal.

Without normal pressure/resistance, the piston can more easily be forced into the master's bore. If the internal seals are worn bad enough, the piston can go right to the end of the bore.

A vacuum booster is just a sheet metal shell that contains two chambers which are separated by a flexible diaphragm - which offers very little resistance to application of the brake pedal.

There is a push rod that goes from the brake pedal arm, enters through the back side of the booster housing. Inside the housing it passes through the diaphragm and operates the control valve. It then exits at the front of the housing and extends into the master cylinder.

Bottom line is: if you have excessive pedal travel, it is because you are not developing design hydraulic pressure in the braking system. This could be caused by a failed MC, leaks in the external circuits or air in the system - but not by a malfunctioning vacuum booster.

I agree and would change the master first then see if the booster is bad. They do make a wooshing sound when the pedal travels more than usual.

 

[kdo58]

my new brake booster make no wooshing noise at all. the old made a loud wooshing sound, since i replaced mine the braking difference is huge.

 

[Georgeb]

my new brake booster make no wooshing noise at all. the old made a loud wooshing sound, since i replaced mine the braking difference is huge.

Sweet. Glad it worked!

 

[firebane]

I am going to have go with a failing master cylinder because of this:

The pedal will go all the way to the floor.


Even without power assist (i.e. with the engine turned off), the degree of brake pedal travel should not be effected. The vacuum booster only serves to reduce the amount of force required by the driver to apply the brakes - it does not limit/reduce push rod or master cylinder piston stroke.

If the master's seals/cups are failing - and the brakes are applied - the result will be insufficient pressure developed at the master's outlet ports and in the external hydraulic circuits. Additionally, the piston will offer reduced resistance to the push rod/brake pedal.

Without normal pressure/resistance, the piston can more easily be forced into the master's bore. If the internal seals are worn bad enough, the piston can go right to the end of the bore.

A vacuum booster is just a sheet metal shell that contains two chambers which are separated by a flexible diaphragm - which offers very little resistance to application of the brake pedal.

There is a push rod that goes from the brake pedal arm, enters through the back side of the booster housing. Inside the housing it passes through the diaphragm and operates the control valve. It then exits at the front of the housing and extends into the master cylinder.

Bottom line is: if you have excessive pedal travel, it is because you are not developing design hydraulic pressure in the braking system. This could be caused by a failed MC, leaks in the external circuits or air in the system - but not by a malfunctioning vacuum booster.

Leaking master cylinder usually ends up with fluid inside the booster which breaks down the rubber causing a brake booster failure.

I had to replace both on my truck.

 

[gsb77]

I've ordered the master and booster, will update after i get them replaced.

 

[1987 GMC Jimmy]

Wait, so a woosh sound when you press the brakes is abnormal? All the old cars and trucks in my family make woosh sounds when you step on the brakes including mine.

 

[firebane]

Wait, so a woosh sound when you press the brakes is abnormal? All the old cars and trucks in my family make woosh sounds when you step on the brakes including mine.

No its not normal and that there is a problem somewhere.