test yourself
 

Something of my own actually broke.
Why not test your diagnostic skills and see if you can figure this out. I will post the final results when I fix it. Feel free to ask any questions that will help in diagnosis.


The story:
2004 supercharged 3.8L Pontiac grand prix with 230 000 kilometers. background info... the car is mint mechanically, no dtc and drove perfect until today. The caliper pins and brackets were cleaned and lubed last year. brake fluid flushed last year.

Here are my symptoms.
driving off from a stop it feels like the brakes are dragging.
Stopped, idling with foot off the brake, the car will not roll forward without stepping into the throttle. The brake pedal has no freeplay and the pedal is hard. Not hard to apply; hard feel right as soon as you apply the pedal. the booster is working properly. After a short 2 kilometer trip all 4 brakes were hot. no pulls or burnt pad smell but I have not driven it far.


What is the answer?

My answer is to always ask Tech2 cuz he's the smartest guy on this forum lol!!

Although you state booster is working properly I still vote one possible cause to be return spring in booster is weakened allowing vacuum to overpower it. Not knowing the mechanics of the linkage this may or may not allow for loss of pedal free-play. An outside possibility could be the return circuit from ABS (valve block?).

Red...lol. Its always good to see what people say.




Golem, I like it. that is one possibility I did not consider. Seeing that it is supercharged, there may be a vacuum pump to power the booster during boost conditions. Maybe I will disconnect the vacuum assist and see if the pedal free play returns. I may disconnect the master from the booster and see what happens.

Keep in mind any time your foot is off the accelerator a vacuum is being generated. Unless the vehicle is being driven in race mode (two footin' it) boostability should have no real-world effect on braking.

Although I have a 1994 Prix (a leftover from my days of wrenching Mitsu's at a multi-point dealer) I'm not sure of the GTP brake system but am betting if they felt it required a source of constant vacuum it would likely be a reservoir with check-valve.

After you've done that short drive, are all the clappers/rotors inordinately hot, or, just one or two ?

Another "test" which may be revealing...
After that short run, without changing anything, leave it running.
Then drive it up on a rack/or however you can slip under, and SLIGHTLY quickly open/close a brake bleeder on front and rear.
See if you get a strong spurt out of one the other or both.
If it's localized to one end or the other, that'll get you closer to the problem.

I guess I need to start ruling stuff out.


Next week I will test drive and measure rotor temp, hoist it and spin each tire, push all pistons back if any appear to be dragging. cracking a bleeder is good point.
I believe at this point all 4 calipers are applying but I have yet to do anything to verify this.


One moment it was normal and the next start up, something instantly felt wrong. the pedal feel and car not wanting to move without more than normal accelerator application.




It appears to be a normally vacuum booster with no additional assist. The new turbo cruize's have a vac pump so it made me think this may have one also...but apparently not.


the esc system does use enhanced programming set up for this car. the abs pump cycles once as a system check. It is continuing to perform this check as usual. no dtc are set in the ebcm.

id do a decent drive measure rotor temps then rack it and see if each tire spins freely for a start point. Kind of surprised no one has mentioned a master cylinder

you may win the prize!

Now I'm totally :confused: !

The way in which the OP reads I wholly assumed this was an out-of-the-norm failure already causal identified and corrected. This thread is not simply a "test your knowledge" prompt?