There is a problem.The rear-wheels braking unevenly.
ACDelco 25852935 GM Original Equipment Rear Brake Proportioning Valve and ACDelco 21994163 GM Original Equipment Rear Brake Proportioning Valve interchangeable?

 

Uh, why would you be looking for a proportioning valve?

As far as I know you don't have one on your car. The ebcm does that job. And if you did how would it get broken?.

Have you tried adjusting the rear brakes?

 

If it's drum rear brakes adjust them as a Don stated!

 

Give yon thread below a look....

https://www.chevyhhr.net/forums/brak...ocedure-36754/

Uneven braking from side to side just about screams "Adjustment Required" or a leaking wheel cylinder soaking the shoes, and you'll find a duff wheel cylinder during the cleaning and adjustment.

P.S. the OP has an '08 LS per his profile

 

I guess a 2008 LS could be without ABS. 21994163 is listed as the correct part. I don't remember how to get interchange numbers. Seems to ONLY fit 2006-2008 HHRs.

25852935 fits ONLY 2009-2010 Cobalt and G5.

I guess it's based on weight distribution.

I don't remember hearing of one needing replacing.

 

The brakes are adjusted. The car without ABS. The parking brake works well. Pads, brake hose, brake cylinder replaced. All new.

 

I doubt it is the valve, since you obviously were able to bleed the rear cylinders. The valve serves to reduce the force to the rear brakes. I would think if it went bad the rears would lock up. If it got plugged up with something then I could think about the rears slacking, but if you could bleed them, that would rule that out.

How about pulling the valve out and forcing some rubbing alcohol or Vodka (whichever is cheaper) through it to clear out any gunk and replace the brake fluid while you are at it.

 

Thank you. I will try to flush the system and replace the brake fluid. Brake cylinders do not leak, a system without leaks.

 

You replaced a wheel cylinder, probably because it leaked? Fluid leaked out, air leaked in. If the brakes were not bled properly to remove air from the lines, that would cause problems.

If you replaced both wheel cylinders, maybe one side is bled properly and the other side is not?

And you replaced one or more hoses also, more chance for air in lines.

 

In-depth audit showed that 1 contour does not work. The front left and rear right does not wheels shows the operating parameters.
This master cylinder?