Anti Freeze turned brown.

When I bought my 71 Valiant slant 6. The car had sat for 15 years. So I know rust and stuff had probably settled into the engine,.
The radiator, heater core, water pump, all new hoses, thermostat. Everything has been replaced.
I put one of those little flush kit caps on the heater hose. And kept pumping water through with a garden hose, with the car running, until it ran clear. I put all new 50/50 anti freeze in it. The car doesn't get driven often. But I do use it once a week, for about 10 miles. It has been good for about a month. Yesterday I popped the rad cap off, to check the antifreeze, and it is brown again! ugh!! Like a brown water.
I was told not to use the chemical flush, that it may take debris loose and clog things. Any ideas? This is frustrating!