orange/brown gunk in cooling system

[Ameer Antar]

Hi All, I've got a new mystery for you. This is from my '85 5kt with less than
7k since restoring it...
 
So I had to replace the radiator fan switch this weekend because the low-speed
part flaked out. When I was in there I noticed some orange/brown mud on the
sensor and in the radiator hoses. The coolant also looked the same. I was
surprised because I replaced the coolant with the blue stuff in October.
Everything is new in the system, except the reservoir. The engine was rebuilt
and first run in June, and I replaced/cleaned out all hoses, even the
transmission cooler.
 
I doubt the fan switch dying is related, but it seems a little too young to
crap out like that. In a way, I'm glad it did to make me notice the coolant. So
what could be causing the coolant to turn brown after only a few months? The
only thing I can think of is in Sept. I bent a valve, and after replacing the
head using ARP studs, I took a little too long to retorque the studs. I noticed
that coolant was leaking out under the head. So could exhaust getting into the
coolant cause that? No real issues with overheating or frothing reservoir. No
water in oil, or white smoke in the exhaust.
 
This weekend I flushed the entire system really well with a garden hose. I also
ran the engine with the hose connected to the engine coolant inlet until the
water came out clear. The coolant is looking nice now, but I have to keep an
eye on it. Is there any way to tell if exhaust is getting into the coolant or
should I be looking for something else? Thanks in advance.
 
-Ameer