Q: My 2000 VW Passat 1.8 turbo recently was flushed to try and clear up heater core, but now my coolant is running out quickly

So I recently refabricated my engine head and redid the gaskets which made my car run great, but after sitting my car got no heat which I could only trace back to having a bad heater core. We tried flushing the heater core with a garden hose and water from the house. This made heat come back momentarily while we high reved the car, so we topped of the coolant and drove the car again. After driving about 30 minutes I'm having to top off the coolant once again which I just basically thought maybe there was a bubble when we first filled it no big deal, but then again after 2 days, driving it maybe 30 mins to an hour a day I needed to top off the coolant again which makes me very worried as this could mean I have a hole in one of my many coolant tubes or a leak in the actual heater core itself. To potentially save me $1,200 I'd like a second opinion from an expert on just what I should try next.

My car has 180000 miles.
My car has an automatic transmission.