what cause a soft brake pedal

Many folks confuse (SOFT) with (LOW), so which one do you (REALLY mean)?

Rule out below, and post back results.

Air in brake lines
Brake pad issues
Brake lining issues (if applicable)
Fluid leaks
Caliper issues
Wheel cylinder issues (if applicable)
Flex hose issues
Master cylinder issues
Drum brakes out of adjustment (if applicable)

For (VEHICLE SPECIFIC) information post:

Year
Make
Model
Exact problem you are having

sounds like you need to bleed the brakes.

inspect brakes for a fluid leaks. especially the rear brake wheel cylinders (if you have drum brakes on the rear).

clamp off all brake hoses, if pedal gets rock hard, problem is air in calipers or lines, if still soft, try bleeding or replacing master cylinder.

Andy,

Are you aware of the specific service procedures for servicing "ABS" brakes?

Did you know that (NOT) following these procedures can damage the "ABS Module"

WHAT HAVE YOU RULED OUT FROM THE LIST I POSTED !!!!

Is the "ABS" light on?

NOT A BRAKE BOOSTER ISSUE. (IT NEVER SHOULD HAVE BEEN CHANGED) !!!!

Did you "Bench Bleed" the master (BEFORE) installing it?

thanks for the help. I have my answer

You're welcome. Glad I was able to help.

let us know if bench bleeding the master cylinder and then bleeding the entire system helps.