It was maintenance time and I replaced the brake pads, calipers, rotors, and then the master cylinder (thought that'd help w/ the soft pedal) on my '99 Grand Prix GTP. I have done other brake jobs in the past on other GM vehicles with no issues whatsoever. This time around though, I can not get the brake pedal to stiffen up once the car is turned on. I have bled the line numerous times with the aid of my helper, thinking maybe air was trapped in there somewhere, but the fluid is coming out fine with no bubbles. I was told that I may have to purchase an ABS scanner to reset the ABS? (My ABS light has not come on once during this process so I am uncertain that's the cause)The brake pedal is very firm when the car is off, and all parts are the correct ones and are working correctly (calipers are grasping, etc), but as soon as I start the car the pedal goes all the way to the floor and after pumping it a few times it still will not firm up. Am I missing something here? I am really lost as to what the issue is, and no one I know has any idea as to what is the problem. I've checked the line for internal leakage/bulging and that's not a problem either. If anyone has any suggestions/ideas as to what may be causing the extremely soft brake pedal, it would be appreciated. I replaced the mc thinking that was the issue, it wasn't. I am just hesitant to buy a pricey ABS scanner since no one rents them in my area and it may not be the solution to this problem. Thank you!!!!!

Did you bleed the brakes starting from the furthest away from the master cylinder and so forth?

Bleed your brakes in the following order:
1st - right rear
2nd - left rear
3rd - right front
4th - left front

Be sure to keep an eye on your fluid while bleeding your brakes.

**Edit** I noticed that I had already mentioned in my first post that yes I did bleed the brakes, but thank you for your advice. I had a helper with me as well who pushed the pedal down and yelled "floor" and kept it pushed down as I bled the line, yelling back "closed" when it was time to remove his foot from the pedal.

Yes, I bled the brakes countless times on friday, saturday, and sunday to the point that I went through quite a few bottles of dot 3 just in case. also did it in the correct order, I have done many a brake job in the past including other GM vehicles; in fact I replaced the pads, rotors, and calipers on my 03 Tahoe with no issue whatsoever early last week. I was told I may need to purchase an ABS scanner to reset the ABS system on the Grand Prix. The only problem is those scanners run betw. $200-$500 and no place rents them out. I have been trying to find a cheaper one to no avail. I am at a loss as to what to do.

It sounds like you got a bad master cylinder. I've seen that happen a few times where the pedal will firm up with the car off, but with the extra assist with the car on, it's enough to blow past the internal seals. Might try swapping that out.

Ya I learned that the hard way.
If you bleed the brakes pushing the pedal to floor, you pushed the diaphram too far in the master cylinder which in turn has caused your master cylinder to take a dump.
Time for a new one or refurbed one, sorry.

This is how I've always bled my brakes and it hasn't caused a problem, including on my GP... so how are you SUPPOSED to bleed the brakes then? Vacuum or nothing? :rolleyes:

That's how I've always done it as well. Never had a MC go bad. I've had brake systems take forever to bleed. Whenever I changed a caliper or line it took forever to properly bleed the system. You said you replaced the calipers. I'd leave the line on remove the caliper and rotate it many different directions that the line will allow to get any air that maybe trapped moved to where the bleed port is. Then re-install the caliper and proceed with the bleeding again.

Your MC still maybe bad but I'd do this prior to spending money on a part you might not need.

Have you ever used a vacuum bleeder? Harbor freight and autozone sell them for under $30. I have had the best luck using these, plus you can bleed by yourself. You fill your reservoir and leave the cap loose. You apply about 10 PSI suction to the bleed port. You loosen the port and it quickly pulls the fluid thru.

http://www.harborfreight.com/brake-bleeder-and-vacuum-pump-kit-92474.html

What would be the odds of having two bad master cylinders though? I thought maybe it was bad, so I had replaced it, and to no avail it did not help the problem at all.
72ChevyMan - that was the exact vacuum bleeder I bought at Harbor Freight last weekend when I was first started having difficulties, it ended up breaking on me, a lot of other parts I've gotten from Harbor Freight have done the same thing (which explains their low prices and Don't Ask return policy haha). I still give the place my business I just don't expect things to work the way the should (kind of like the TJ Maxx of hardware stores). I'd like to thank everyone for their advice and time, I guess I'm just gonna keep on bleeding and bleeding and bleeding...it'd be just my luck that possibly a tiny bit of air is caught in some nook or something.

I had the same problem, the difference is mine happened after I changed a caliper. After fighting this problem for a week and four quarts of brake fluid I took the calipers lose from the car and pointed the bleed port up. With the rotor installed and a bleeder tube running back to the reservoir I pumped the brake pedal about 50 times for each caliper. I never noticed any extra air bubbles but the brakes work great now. hope that helps

I forgot to add that I did the power bleed with the scanner but didn't make any difference except using massive amounts of brake fluid.

It might be the brake booster, I had that happen once while visiting family on vacation. Costed about $530. at the dealership.