After a 30 minute drive the engine Coolant seamed to leak directly out of overflow tank. I noticed that the top hose was boiling pretty well too. The antifreeze looked brown/rust but not sludgy. I have noticed it leak from here before, but after recently flushing the system it seemed to have stopped. However, this is the first time I have seen it this color. It did not come close to overheating. Any body have any clues as to why it's leaking brown ****?

 

When you say it's leaking from the overflow tank, is bubbling out?

If the coolant was clean and no other viable contamination, then I would point to a head gasket problem.

What prompted you to flush the block and refill? Was there a problem overheating before or was this just routine maintenance for a poorly neglected engine/truck?

Reason is because, I had a similar problem. Engine never overheated, but the coolant would boil out the overflow. Checked the compression on all cylinders and that was fine. Finally, I hooked up a air line into the #7 cylinder, used about 60-80lbs compressed air and viola! Found the head gasket leaking and allowing exhaust gasses to enter the coolant ports on the head.
That engine was neglected and abused.

Had the heads redone, new head gaskets and never had another problem.

 

Timbersteel- I'm not sure if the Coolant was bubbling out of the overflow or not. The main reason I decided to flush my Coolant was to replace my thermostat and hoses. I did have a very small leak coming from what looked like my water pump as well. Once I had flushed and replaced thermostat both leaks stopped, until just yesterday. This time with a brownish/rusty color. I hope it's not a head gasket issue, but if so why would it change the color of the antifreeze? I'm most certain that oil isn't mixing with it. Rust in radiator?

 

Rusty, neglected radiator and/or engine block sounds feasible.

I usually use Prestone radiator flush or blue devil and then some vinegar and run it several days. I couldn't believe how much crud was in both.