If you have to add a quart or more of coolant when bleeding the system each time, and you don't see any coolant leaking under the car; then there probably is a leaking head gasket; which sucks coolant into a cylinder and pumps exhaust gas into the cooling system. Take the cap off the coolant reservoir, and see if you can smell exhaust gas at the reservoir opening. If you can; that would mean there is a head gasket leak.
The way to repair a leaking head gasket is to remove the cylinder heads and have them inspected for cracks or warping by a machine shop. If the heads are not severely damaged; they can be remachined. Damaged heads would have to be replaced. This work often costs $1,000 or more.
If you are not losing extreme amounts of coolant; it often is possible to seal a head gasket leak chemically, without taking the engine apart. There are several products sold for this purpose; but only one of them works well enough for me to recommend. It is called Irontite Ceramic Seal. Some serious parts stores carry it; but if you can't find it locally; it can be ordered from the manufacturer: http://www.irontite.com/
. I would not use Irontite All Weather Seal for this. The Ceramic Seal will not work in anti freeze; so the cooling system must be thoroughly flushed out and filled with plain water before using the Ceramic Seal. Follow the directions on the bottle completely. The sealer should be drained out and replaced with 50% anti freeze-water mixture after driving the car for 2 or 3 days. I have saved several engines from expensive repairs with this product. But if there is anti freeze left in the system; it will not work; so it is critically important to flush the system well, and make sure the heater is set to hot when flushing the system.