This pulling to the left is usualy a sign that your frame/fork isn't aligned properly and wheel dish is a possibility as well. Set the bike upside down on the handlebars/saddle and sight along the wheels. If they aren't in perfect alignment, and close enough isn't good enough then that's the cause of your problem.

This problem is unfortunately common these day's and I suspect that the fashion for vertical rear dropouts is at least partialy responsible. The modern factory road bike I had for a year wasn't paticuarly stable and the cheapy I bought was shockingly out of alignment. My custom steel road bike with semi-horizontal dropouts is spot on though and is perfectly stable with no hands even though it has VERY fast geometry.

I also bought a vintage cheapy childrens bike the other day and its alignment was spot on. Dodgy work just seems to be the norm these day's.

Regards, Anthony