Is200 Brake Failure, Advice Needed Plz

[XK8BMWLexus]

hi all

IS200 2002

help needed to fix my brakes.

drove the car last week to find that the brake pedal has excess travel (brake pedal nearly to the floor before brakes come on). when i checked the brake fluid reservior, the brake fluid was low (most likely a leak in the system somewhere). i topped up the brake fluid and bled the system. but the brake pedal still has too much travel. checked all the pipelines but could only find a very very hardly noticable small drizzle on the drivers side front caliper. changed the caliper but brake pedal still has too much travel. now changed the master cylinder but problem is still there. might add that the caliper and master cylinder were used parts, so i am thinking whether i've ended up with duff parts or should i look elsewhere to solve the problem.

any advice will be very much appreciated

thanks

terry