Noise from brake pedal

in hot days,my 1999 grand marquis brake pedal sinks deeper when applied, giving out a deep “wooosh” noise. On cooler days, the brake pedals are higher and little “wooosh noise”.

Plenty of brake fluid. Is the booster leaking? Air in the brake system? What’s the cure?

The whoosh is air entering the vacuum servo, either the atmosphere vent damper is open/missing or air is leaking past the brake pedal actuating rod seal, this in itself isn’t dangerous it just makes an irritating noise. But if the noise is excessive it could also indicate a leaking servo diaphragm.

The pedal height shouldn’t noticeably vary with temperature unless you live in a high humidity climate and your brake fluid has absorbed a large quantity of atmospheric water or other hydraulic component problems exist.

I’d strongly suggest getting the servo vacuum and brake system thoroughly checked at a reputable shop before using the car again.

Is this related to the use of the AC? On all of my Grand Marquis’s (what is the plural of Marquis?), at a stop, the brake pedal would sink slightly every time the AC compressor would cycle off. I think this was because of the slight increase in engine vacuum when the engine load was reduced.
If this is not what you are talking about, then you do need to have your vacuum booster and master cylinder checked out.

Any hissing noise coming from the brake pedal area indicates a defective vacuum booster. When a booster operates properly you will not hear any hissing noise. The hissing means the diaphram is ruptured and the booster needs to be replaced.

That’s funny, all of my cars give a very slight hiss anytime the brake pedal is moved. (There is no hiss while the pedal is held in any position.) How would a brake booster work if it didn’t allow air in and out as it moved? That’s the whole point of how the booster works.

Not quite - air is drawn into the booster under actuation as the diaphragm vacuum side volume decreases the atmosphere side volume increases - there is an air vent with a felt damper to allow the air in. If the damper is missing, dislodged or blocked with gunge then it’ll hiss. Blockage can also cause air to be drawn past the rubber actuation rod seal (since nature abhors a vacuum) again causing hissing.

The diaphragm might or might not be ruptured, either way it merits further investigation.

Even with the damper, if you listen closely in a quiet car, you can still hear a hiss as the pedal moves. I know this to be true for the 1991, 1994, 1999, and 2000 Grand Marquis in particular.
On the other hand, if the OP is hearing a hiss even when the pedal is not moving, then he does indeed have a problem.

My point is that a hiss while the pedal is moving is not abnormal.

The problem might be that the brake master cylinder is leaking internally when it gets hot.

When a worn master cylinder gets hot, the bore in the master cylinder expands from the heat. This then prevents the worn cup seals from completely sealing against the bore. This then allows the brake fluid to by-pass the cup seals causing the wooshing noise and the brake pedal sinks further to the floor. On cooler days, the master cylinder bore doesn’t expand as much so the cup seals don’t allow as much brake fluid to by-pass them, so there’s only a little wooshing noise and the brake pedal doesn’t sink as much and remains higher.


Scudder and Tester. Many thanks for your detail explanation of the “wooosh” (not hissing) which i think to be the most likely problem/s - the actuating rod seal?
Seems logical that is this why the pedal comes up higher in cooler weather and the “wooosh” less noticeable?.
Brake fluid level is good and i do not see any leaks and am not losing any.
Have taken the car to the dealer and asked him to bleed the system. He said there’s no air in the system but replaced the brake pads and rotor which didn’t helped. Same dealer does not believe its a safety issue while i’m uncomfortable with the noise and the brake pedal sinking lower in hotter weather. Feel quite sure they’d suggest replacement of the booster, master cyclinder if not the entire brake system…calipers, hoses etc.
Vehicle has 101,000 miles and i- the only driver, have it since new.