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How do you mean soft? no resistance on the pedal,spongy or does the pedal just sink to the floor?
Have you actually checked for fluid leaks?
How are you bleeding the brakes? any air bubbles seen in the line when bleeding at all ?
Sounds like you still have air in the system/air pocket or leak possibily...considering you have changed the master "which i asume you bench bled before fitting if it was new" then the problems likely going to be either air pocket trapped in the line or there actually leak somewhere...have the brake flexis been checked to be sure these are not swealing under pressure or brake applied..as well as the fittings on the lines for small leaksat the brake lines and too into the calipers....normally when bleeding we use a pressure bleeder at the reservior ensuring the fluid is topped right up and then once the the pressure is brought up at the reservior usually somewhere around 25psi..then you start off at the furthest away bleed nipple from the master cylinder and work your way round to the closet at the front...sometimes that can work rather then pumping at the pedal continually..something to may try...but failing that..then if it fitted with the ABS then fault could be in the abs pump itself.seals wise either inverted by forcing back brake fluid to fast either due to retracting caliper pistons back or the pumps been run dry..resulting in the soft pedal.....