2005 Pontiac Grand Am intake valve seals

For about 3 weeks my car was losing about a quart of water a week.I took it to a garage for a compression test. They told me it was my intake valve seals. The tube turned green I think. 375.00$ to fix. A week after I got it back it had to have another quart of water. There's no ater under my car, no oil in the antifreeze and no signs of leakage under hood. Could you tell me what the problem is or may be. I travel about 500 or so klms a week.