Automobile(s): 1993 Coupe Deville, 2009 XLR-V 1 of 6 Crystal Red
Orange County, CA
Re: Brown sludge?
It might be the stop leak dumped in it over the years. I have heard GM does not recommend using it anymore, although they still sell it. Make sure the small hose from the radiator to the reservoir isn't plugged. I take mine off and flush it out best I can if I change the coolant. I'm getting ready to change it out and am going to order a new reservoir this time.
I read about this a while back, I forget what it is... I've seen this a number of times even in vehicles that dont take the powder. I believe it has something to do with the coolant vapors condensing inside the tank and forming that brown almost 'foamy' stuff kind of like standing water does in a mud puddle. I dont think you need to worry about it. I usually take the tank out and flush it. I see it more often on vehicles with pressurized tank systems unlike your 4.9's 'overflow' type.