Not sure if this can be diagnosed over the internet, but I was wondering if anyone has experienced this or knows what these symptoms might be pointing to...

The car is a 1994 Pontiac Bonneville SE, and the problem is as follows: Sometimes, not all the times, the brake pedal will fall to the floor during braking. Usually the brakes will perform as normal initially, but then it feels like I'm losing the power assist and the pedal falls to the floor and I have to jam the pedal into the floor to get it to stop, but it will always stop. Note that the problem is intermittent, sometimes it will work fine, other times it will be fine till I stop, then the pedal falls to the floor, and other times I'll be slowing down and the pedal will fall before I get to a complete stop like I was describing above.

I have recently checked the brake fluid level and it is full, I have also bled the brakes to see if there might have been some air in there. The pads and rotors are in good shape as well as the drums in back.

I also took it to an auto shop and a guy test drove it and said that he suspected the master cylinder is bad, but he wasn't 100% sure. He did say that he was pretty sure it wasn't an ABS problem, though.

Seems to me that it might be a problem related to the brake booster, or a vacuum line that's feeding it. Another of my friends that works on cars drove it and tends to agree with me but is still not 100% sure.

So I ask you, the T-dubbers of the Garage what you think -- thanks in advance for any ideas that might be helpful!

10/4/2006 2:12:22 AM

Master cylinder has/is shitting the bed. Intermittent function of the cup seals and/or the check valve in the piston assembly.

If it's a brake booster problem, then several things can occur. One is that the pedal will be constantly sucked to the floor if the internal valving is faulty. Another is that you'll hear a constant whooshing sound when you hit the pedal (rupture in the rubber diaphragm inside the booster. Still another is lack altogether of brake boost.

I'll bet a dozen Krispy Kremes on it.

10/4/2006 2:29:29 AM

^like zxappeal says, it's the master cylinder. Those are the same exact symptoms my old car was having. Luckily, they are very easy to replace. I fixed mine myself after reading some basic online instructions.

10/4/2006 9:35:30 AM

^yep master cylinder

10/4/2006 9:48:37 AM

So, I'm gonna go ahead and put in the new master cylinder tomorrow afternoon...

Y'all are pretty sure about this being the problem?

It's only like a $65 part, btw, only 2 bolts and 2 brake lines to swap it out, only pain will be bleeding all the brakes.

10/4/2006 11:58:41 PM

See my above bet. It's all I will bet, because diagnosing shit over the internet just screams "missing some vital details"...

BTW, make sure you bench bleed the master cylinder first...because if you don't, you'll NEVER get it to bleed.

10/5/2006 1:07:03 AM

Quote :

"Master cylinder has/is shitting the bed."

you sure? i know a dozen k.k. hot glazed doughnuts is a hefty bet but doesn't this car have power assist braking? i would be willing to bet that the unit itself is going bad since he said it happens intermittently. now if he just bought the car and isn't use to the power assist braking, then it will feel like the brakes are going out under hard braking conditions. note he also said that the car will also stop. if the master cylinder was going bad, then he would have to pump the brakes in order for the car to stop.

10/5/2006 9:14:07 AM

Quote :

" if the master cylinder was going bad, then he would have to pump the brakes in order for the car to stop."

Not true in my case... when my master cylinder was going, sometimes I had to pump it, and sometimes I didn't, and then other times as long as I floored the brakes then they would kick in at the very last second. Like zxappeal said, more details are needed.

10/5/2006 9:35:55 AM

well since putting in the new master cylinder it seems that the problem has improved, but it still doesn't feel as responsive as some other, newer cars I have driven. The pedal no longer falls to the floor where I have to jam it into the floor, but I have to press the pedal farther than I think I should have to before the vehicle comes to a stop.

are there any tests I could do to help you diagnose the problem or any questions I could answer??

thanks again guys.

10/6/2006 6:35:30 PM

n/m.

[Edited on October 6, 2006 at 7:00 PM. Reason : ]

10/6/2006 6:50:50 PM

You need to do a better job bleeding your brakes.

10/6/2006 7:22:30 PM

I bled through like a whole pint of brake fluid

10/7/2006 1:03:17 AM

A pint ain't shit. Did you start at the right rear, go to the left rear, then right front, then left front?

I bet another dozen Krispy Kremes (this time with creme filling) that you need to bleed more.

10/7/2006 1:35:23 AM

yeah i went in that order (i have replaced my brakes before and know how to bleed right)

another dozen krispy kremes with creme filling would be nice tho ;-)

10/7/2006 10:00:42 AM

Quote :

"make sure you bench bleed the master cylinder first"

did you?

10/7/2006 11:23:00 AM

try cracking the lines loose at the mc, and bleed it right there at it, someimes you can get some air there thats tough to get out, otherwise, a pint isnt even enough to move the air from the mc to the bleeders through all the lines

10/7/2006 11:48:50 AM

Quote :

"another dozen krispy kremes with creme filling would be nice tho ;-)"

Yeah, like I really need to eat two dozen. But I'll share.

^What Keith said. You need to bleed some more. Seriously. I'm an old fucker. I been bleeding since you were prolly shittin' in yer diapers. And I was pushing the pedal for my dad before you were born!

10/7/2006 2:48:36 PM

i don't owe you shit yet. he said his brakes still ain't right. dammit.

10/8/2006 10:55:17 AM