There is a brown sludge in the coolant. I had a chemical and pressure test done both was ok. Can it be the EGR cooler? OR the oil cooler? A manual does not have a cooler for the gearbox but does it have an engine oil cooler?
Generally a brown sludge in the coolant is from a leak sealant additive. This is probably all that it is since it did pass a chemical test. I would also think it has sealed the leak since it passed the pressure test. Should be safe to drive the vehicle this way without worry.