Urgent! Need help! about brake system. Please help me!

I just changed the front brake pads on my nissan sentra 2005 1.8s. I follow all common steps. After the work is done. I notice two weird things.

1) I could not push brak pedal when the engine is off. However, I could push the pedal couple times right after I turn the engine off. After that the pedal will be hard like a concrete.

2) When the engine is on and the transmission is in P(park) position, I hear a click noise (from my transmission in side the cabin) every time I step on the brake pedal.

What should I do? Please help me! I will be checking my email and this post every hour when I am awake.

I really appreciate all your help in advance.

Thank you,
DRT

After looking over the internet, It looks like I have a bad piston. I recalled that it was hard to push the left piston in and I could not push it in all the way (about 5mm left compared to the right one).

I still don't know why I heard the noise from my transmission (inside the cabin) only when I step on the brake when the transmission is in P position.

Please help!

Still have problem!

I removed the wheel and check the piston but it has no leak. My assumption was wrong. I do not know where to start from here.

Please throw me some idea.

Thank you.

You really need to give people time to answer you.

Everything you have described sounds normal.

When the engine is on, the vacuum to the brake booster assist with the braking. When the engine is off, applying the brakes will bleed off the vacuum that remains in the brake booster and will increase pedal effort.

The clicking noise is the shift interlock. It prevents the gearshift from being moved out of park without the brake pedal depressed.

Thank you for your help 91STT. I also asked my fiend to take a look and he said the car was ok. I never over hear the shiftlock that lound before neither from my sentra or other cars. May be I am being paranoid after I did the brake job. It was my first time doing this tho.

I will change my O2 sensor (1, before CAT) in couple days and I will post some questions if I need help.

Thanks again,
DRT

why are you replacing the o2 sensor?
is your service engine soon light on?
what code are you code are you getting?

91STT Thank you for asking. It is very kind of you. At first, I thought that you could read my mind about changing O2 sensor. I actually told you.

About O2 sensor:
Couple months ago, I started to notice that my rpm was not stable (swing by +-300 rpm) at idle or D without moving. The service engine soon was on last week. I went to advance, the guy told me it is A/F ration problem. I didn't see the exact code. I went to the dealer right after and force them to scan my car for free. The dealer just put a new fuel pump (and fuel switch) in Feb 2010 (under warranty) and I blamed the problem on their work. The result from the reader was [A/F sensor1 (B1), P1278]. The sale person at the dealer knew that I would not do the work with them. The car was parked outside when he gave me the scan result. I asked him for the price anyway. He told me $220 for the O2 sensor and $140 for the labor. I shopped around and bought the sensor from advance for $98. It looks like the sensor is on the engine header. I will have to fine the wrench that will fit in the area or remove the metal sheet that cover the header. I will have to double check the location of the sensor again before (if) I do the work.

I would appreciate your suggestion about this. I can always return the sensor if I don't need one.


About the shift interlock:
My car did not make that noise before. Right now, the shift interlock makes pretty loud noise. It sounds like you tap a pen on a table. I did not recall that I heard the noise when I test drove my car after the brake job. I went to goodyear after to let them flush power steering fluid and clean the fuel injectors (to make sure the inside parts are cleaned bofore I install a new O2 sensor). I started to hear the tap noise when I drove my car on that night.

I look through under my hood and saw some hand print here and there. The battery terminals were cleaned by water but there were not disconnected. Would it be possible that I have some loose connections that can couse the shift interlock noise? (The mechanics might have knock some switches off.)

How do I make the noise go away? Many people told me that it is ok to have some noise from the interlock, but it is loud and I am worried. I have a repair manaul from nissan. I saw that the swicth is located on the driver side under the shifet lever. How hard to get in there and how can I fix the problem? Can I just adjust the adjuster on the interlock system? If it is 100% safe to have the noise, I can live with that. I do not want to touch any thing that is close to transmissionm system.

I will be awaiting for your reply.

Thank you
DRT

FYI:
-Nissan sentra March 2005 (New) 60k
-Fuel tank recall (fixed October 2005, the gas mileage went down from 35 to 31 miles/gallon highway)
-December 2005, Replace rear drums (warranty)
-April 2008, Starting problem when the weather is hot (engine can be cool or hot). It cranked but would not start right way. I always got lucky after several times of trying. (never have to tow)
-In Feburary 2010, high pitch noise from fuel pump. Advance told me it is the fuel filter. Fuel pump has 5years/ 60k warranty so I went to the dealer and they replaced the fuel pump (warranty, the gas mileage came up from 31 to 34 miles/gallon highway). The pump also came with the fuel switch (or whatever name it is) and that fixed my starting problem (dealer told me). The switch was not under warraty but it came with the pump so it was free. (Lucky me!)

Before replacing the O2 sensor make sure there are no vacuum leaks anywhere in the intake tact after the MAF.

I have a 2004 and the shift interlock is only noticeable to me when I am paying attention to it. There is nothing that can be done to quiet it.

The AdvanceAuto where I'm at will sell you the O2 sensor socket (it's a special socket w/slot cut in it for the sensor wires). When you're done and return it, they give you your $ back.
Hope this helps.