soft brakes and brake light comes on-help[

Did you bench bleed the master before installing? Kind of an important step which many people forgo for various reasons. When you don't do it, you almost always have to re bleed it. You can get a cheapo bench bleeding kit at the parts store and do it while its still on the car. Just need a helper to pump the brakes while you watch the fluid. Can be messy so get some rags and cover up any paint that can be affected by splashing brake fluid.

The brake light comes on because there's a difference in pressure on one side of the system. There is a little valve or check ball in the brake switch. A pressure differential makes the valve move to one side or the other where it makes electrical contact and turns on the light. Almost always air or a leak.

Usually, a bad booster will give you a rock hard pedal with barely any travel. Lack of power assist is just that - no power assist.

You could take the line off the booster and check it (booster) with a vacuum gauge to see if it holds. If the booster is leaking vacuum, you can sometimes hear it under the dash, kinda like a squeak.

You could possibly have a leaking diaphragm inside the booster. If so, you may not see any external leaks but fluid can sometimes get into the chamber behind the master. You can try to disconnect the master from the booster (with the lines still connected to it) and see if there's fluid around the back seal or leaking down the inside of the firewall.

I would try gravity bleeding. Take the cover off the master and crack all the bleeders open. Make sure they start dripping freely. Go have a sandwich and and re check afterwards. While the bleeders are open just make sure to keep the master full. Brake fluid gets slippery, put something under the bleeders to catch the fluid so you're not tracking it all over.