Brake grabs in front wheel

This is a 4.2L engine, when driving, braking is very hard to control because front brake grabs and skids, even when is applied slowly and gradually, seem like is mostly on left front wheel, but no skid in the rear. I replaced a brand new master cylinder brought from ford dealer, same problem again but a little better than before, I bleed it really good. So if any experienced ford mechanic know what cause this brake problem, please give me some advice. Oh, no ABS light were on, I was suspecting the brake booster, or is it possible the calipers, but the caliper wont skid that bad. Is there a proportioning valve on this vehicle? Please help, thank you!