Brake Issue I Can't Understand

[ColoBonne]

My 2005 SE has a braking issue I don't quite understand. When slowing down to a stop and I lessen the pressure on the brake pedal to make a nice smooth stop the pedal falls away so that it's just above the floor. If just slowing down for traffic, the pedal does the same thing, but if I lift completely off the brake and then apply them again the pedal height is back to where it should be. For the last few days it hasn't been doing it, but today it's back to doing it again. The system isn't losing fluid and I'm at a loss to understand what's happening.

[96 SSEi]

hi, have you done any brake work before the issue - if yes, I would suspect air in the system - bleed system

if not, could be a number of issues - check this site pl............https://www.2carpros.com/articles/brake ... -the-floor

[maxi426]

Sounds like master cylinder leaking internally. Seals will leak under light apply pressure

[nos4blood70]

Sounds like master cylinder leaking internally. Seals will leak under light apply pressure

I had your same problem and it was solved by changing the master.

[MKMike]

In the absence of hydraulic leaks, a sinking brake pedal indicates a failing master cylinder .
It will not improve and will only get worse.
You probably noticed the problem is worse when the air temps and brakes are cold.
Want proof that it's a bad master? Read this: http://freeasestudyguides.com/master-cy ... pedal.html

Replace it now before the pedal goes all the way to the floor and the car doesn't stop when you need it to.

[ColoBonne]

Is replacing it doable by me or does it require something special to bleed the brake system afterwards?

[MKMike]

It doesn't require super expensive tools but you would be wise to use flare nut wrenches and not open end wrenches when loosening brake lines.
If you can bleed brakes and turn a wrench, you can replace the master cylinder.
Buy a NEW, master cylinder. Do NOT buy a remanufactured or rebuilt unit.
Rockauto has good prices and gives forum members a discount (code in their Pontiac Bonneville Club vendor's forum post).
Centric has been an affordable and very reliable brand and the Raybestos one they have on closeout should be good, too.
If you're in a hurry, Amazon often has parts at decent prices, especially if you're a Prime member.
Buy locally if you can't do without driving it for a couple of days.

You will save a great deal of time if you "bench bleed" the master before installing it.
Youtube has some good videos about that, such as this one
https://www.youtube.com/watch?v=vOTkVAeU_n4

You will then bleed the master cylinder again, after installing it on the car.
This guy knows his stuff and makes decent videos (though his preferred bench bleeding method is a bit tricky if you have smaller fingers---and you don't need to swap the reservoir on a Bonneville master)
https://www.youtube.com/watch?v=trotrlIG2sk

As he says in the video, you may get away with just the bench and on car bleeding of the master cylinder, rather than the entire brake system.
The advantage of bleeding the whole system is that you are flushing out the old fluid, which is good for the brake system.
It removes moisture from the system, prolonging brake line life.