brake response is non linear ???

The first thing that comes to mind is a bad wheel cylinder. When they go bad, they tend to do this, and lock the wheel with the bad part. If it is all of the wheels , it could be a faulty master cylinder or a system that needs bleeding ( always go farthest from master to nearest- rt rr, lft rr, rt frt, lft frt). One final thing that affects all the wheels is a bad proportioning valve, but these hardly ever fail. Just curious, have you modified the system in any way, or is it completely stock? Also, have you ever replaced the brake hoses? If not, now would be a good time as they tend to dry rot and fail in older cars.

The pads are heating up and it changes the co-efficient of drag. Try the test again with breaks fully warmed up. How does the pedal feel? Does it stay hard and up or go down mushy? The pads you have may issues. I would think if you had a bad master you would have the opposite effect of what you are experiencing.

I would pull all 4 wheels and have a good look at all the brake components but I think that the braking response that you so clearly indicate with your graph might be normal with all G-Bodies. When I first started driving my 1979 Elky I thought exactly the same about the brake response/brake pedal effort. Other than normal wear and tear I didn't find any problems and I learned to sort of live with it after I drove a couple of other G-Bodys and noted that the brakes worked similary. Since then I have upgraded the brakes and I am much happier with the pedal feel now. If you get a chance drive another G-Body for comparison.

evan said:

As far as I know, the system is still completely stock. The car is new to me but everything else on the car is stock so I don't imagine they modified it in any way.

So, would you suggest replacing the brake lines and bleeding the brakes as a first step? I've never bled brakes before, is there a how-to somewhere I could read?

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The brake hose suggestion has nothing to do with the problem, it's just a good idea because they tend to dry rot over time. As for the master, the only real way it would do this is if it were improperly sized.( out of another application) Brake bleeding goes a little like this: get to the bleeder screw on the wheel in question and get a helper to depress the pedal. Once it is depressed, open the bleeder screw and let fluid gush out. Then, close the screw, and have the helper let off the pedal once the screw is tightend.Do this a couple of times until nothing but fluid comes out( no air). If changing the fluid, do it until clean fluid comes out. If changing all the fluid, you can also gravity bleed it first, by opening the bleeder and letting gravity take it's course until fresh fluid comes out, then follow the other procedure. If you do decide to change the hoses, remember to put new copper washers on the front caliper banjo bolts or it will be more likely to leak.

I'd pull the drums and see if the wheel cylinders are leaking in back too. If so, new ones are only $10 ea, so go ahead and replace them and the shoes.Remember to leave the parking brake off when trying to remove the drums.

if the wheel cylinder is leaking you will see it on the back/bottom of the drum. and that will cause it to lock up, if the cylinders are good the brakes are probably just set up too high. they tend to lock up if they are.

"What did you do to upgrade yours?"

On my 1979 El Camino I replaced the front calipers and rotors with SSBC dual piston calipers and SSBC slotted rotors. I also used metallic/ceramic rear shoes with the larger wheel cylinders from the Chevy Blazer. Lastly I replaced the stock rubber flex hoses with braided SS hoses from Russell. The improvement is very noticeable but if you want the eyeball straining stops that only a big brake upgrade will give you, replace the front rotors with 13" large rotors and install rear disc brakes. Be prepared to make a large investment in your brake system upgrades, at least $2000 or even more. My upgraded brakes cost me about $800, but I did all the labor myself.

mrapii...for the rear wheel cylinders you said you used the Blazer cylinders. I used the cylinders for an S10 non power brake which come with larger diameter pistons and fit the G body back plates. Do the Blazer cylinders fit the backing plate in the G body?