04 GP, rear brakes lock up under hard braking

**mars5l** · 04-01-2018

Ive always had this issue and was wondering if there was a factory tsb related to it. My car is a non abs car, any kind panic or unplanned hard braking the rears lock up but not the fronts. It's bad to the point its like pulling the ebrake and im going sideways at 55 mph. I can never fully utitlize the fronts because the rears are already sliding. Had different pads over the years and brakes are bleed. It feels like its a bias issue or weigh transfer issue. Any thoughts?

**SaukRapids** · 04-01-2018

Are the parking brake levers stuck? Common problem. They have to be toward the front of the car to be off, iirc.

Otherwise, try bigger fronts with different components like slotted and/or drilled rotors.

**cheatah faheatah** · 04-01-2018

^^^^ This sent me off the highway, through a plow bank & into a snow covered muddy ditch last winter. 06 GP base, also non-ABS. I installed new front rotors and front pads of a better grade than my rears. Fronts do like 80% of the stopping "work" , so Premium pads up front & "ElCheapo's" in the back can help. Some weight in the trunk ( tools, real jack ) helps too.

**mars5l** · 04-01-2018

I currently have new rotors up front with Hawk HPS 5.0 pads and regular fresh Ctek cermaic pads in the rear. And yea I have the fbody front calipers up front with new stainless lines from zzp.

**BillBoost37** · 04-02-2018

At 14 years old and different pads/rotors etc later I'm pretty sure no TSB would exist for that. Look under the car at the brake lines by the rear left tire, do you see any proportioning valve or round thing in the line that isn't just a connection point? Do both wheels lock or just one?

Autozone shows one, but I'm not 100% sure your car has it. I believe that the non-ABS cars had it to keep this issue from happening. Sounds to me like you are getting too much fluid back there vs the fronts.
2004 Pontiac Grand Prix Brake Proportioning Valve

**mars5l** · 04-03-2018

The link doesnt send me anywhere. I looked around the master, I dont see anything that looks like a proportioning valve and don't remember one on the rear. I know with my car there is 4 seperate lines off the master to each wheel. It definitely locks up both rear tires, twice ive went sliding off into the grass in a panic stop from high speed. I was hoping GM made flow reduction fittings that would attach to the lines at the master. I'm not really fond of having to buy 2 of those adjustable proportioning valves you get from road racing catalogs and trying to adapt and tune for each wheel

**FordMan77** · 04-03-2018

The link worked for me, so here is the info Bill posted:

Autozone: Dorman Brake Proportioning Valve

part # 905-952 w/4 wheel ABS

**mars5l** · 04-04-2018

The link didn't work from my phone but works from a computer. I see what it is, but according to the autozones site its not for my car, its from the previous gen car. I will check next time I put my car in the air if there is something similar or an area on the lines I can add one in.

**mars5l** · 04-15-2018

So this is where I COULD possibly add the Dorman proportioning valves, but id have to know if the threads are the same and the flare type since Dorman has several listings but nothing specific for my car. That or I can order Baer Brakes Proportioning Valves 6801276 - Free Shipping on Orders Over $99 at Summit Racing from Summit and have adjustablitiy, will just have to spend time dialing it. Though even with this, id still need to know what fittings and flare type so I can order the correct ones before taking it all apart. Dont want to have to take it apart to find out before hand and end up having to bleed the brakes