Two Brake Questions

[SiVisPacemParaBellum]

I have two questions about the brakes on my 2006 Jeep Liberty.

Question 1:
The brake pedal feels like it "gives" after a small amount of time (1-2 seconds) of constant pressure. This seems to happen all the time. What is this? Is this some sort of "braking assist" feature, or am I experiencing a problem?

Due to the nature of how I drive (i.e. not aggressively), I usually do my best to apply an even amount of pressure to the brake as I approach a stop. However, because of this "giving" that I'm trying to describe, I actually feel like I initially press the brake to begin the stop, and within a second or two, I actually have to release the brake somewhat, because of the "giving". If I keep the pressure perfectly even with my foot, I actually end up slowing down faster than I would like, because as the pedal "gives". I can tell more pressure is actually being applied, because I'm slowing down at a faster rate than before the "give" began.

Question 2:
When I apply the brake, there is a sound coming from the dash. The best way to describe the sound, in one word, is "hissing". The "hissing" sound is somewhat muted, meaning it doesn't actually sound like it is really air, though I suppose it could be. (It actually reminds me of a sound I used to hear when lowering a jack my father used to own years ago; it sounds to me like pressure being released.) What is this? Is this part of the brake system's normal "hydraulics", or am I experiencing a problem?

Thanks in advance.

 

[ms5490renegade]

do you have ABS

 

[SiVisPacemParaBellum]

ABS - Yes

Yes, mine has ABS.

 

[ms5490renegade]

that may be what the release on the pedal is

 

[tjkj2002]

For #1 that could be the vacuum assist kicking in,some early KJ's have had known master cylinder internal leaks.There is a easy way to test that,see below.......

DIAGNOSIS AND TESTING - MASTER
CYLINDER/POWER BOOSTER​

(1) Start engine and check booster vacuum hose
connections. A hissing noise indicates vacuum leak.
Correct any vacuum leak before proceeding.
(2) Stop engine and shift transmission into Neutral.
(3) Pump brake pedal until all vacuum reserve in
booster is depleted.
(4) Press and hold brake pedal under light foot
pressure. The pedal should hold firm, if the pedal
falls away master cylinder is faulty (internal leakage).
(5) Start engine and note pedal action. It should
fall away slightly under light foot pressure then hold
firm. If no pedal action is discernible, power booster,
vacuum supply, or vacuum check valve is faulty. Proceed
to the POWER BOOSTER VACUUM TEST.
(6) If the POWER BOOSTER VACUUM TEST
passes, rebuild booster vacuum reserve as follows:
Release brake pedal. Increase engine speed to 1500
rpm, close the throttle and immediately turn off ignition
to stop engine.
(7) Wait a minimum of 90 seconds and try brake
action again. Booster should provide two or more vacuum
assisted pedal applications. If vacuum assist is​

not provided, booster is faulty.

As for #2 if you have no vacuum leaks that is the ABS pump kicking in,don't change your braking habbits,pumping the brakes in a ABS vehicle defeats the ABS purpose and can cause accidents(has been proven).

 

[JeepJeepster]

If the ABS is kicking in then he will certainly feel the pedal vibrating and hear the pump kicking on. Its very easy to feel that and I do not think thats whats happening.

You are probably just feeling the vacuum assist kicking on like tjkj said. I also notice my pedal doing that if Ive been driving for awhile and haven't used the brakes.

When the jeep isnt running, push the pedal kinda fast but not hard. You should hear the same hiss. If thats the same hiss youre hearing then its normal.

 

[tjkj2002]

When the jeep isnt running, push the pedal kinda fast but not hard. You should hear the same hiss. If thats the same hiss youre hearing then its normal.

That would NOT be normal,that would mean a vacuum leak which is not good.

 

[JeepJeepster]

Ive always heard the pedal hissing in a vehicle if I push it. Even if its running or not running.

If I wasnt stuck in a dorm Id take a vid.