I have a 1979 Cadillac miller meteor olympian hearse.
I have replaced the Master cylinder and the booster (came as one to save time) as I was having problems with the brakes not working.
All pipes, calipers and pads etc are fine.

I fitted them to the car and bled the breaks and the pedal was stiff as expected.

But. As soon as I turned on the engine the pedal went soft again.

I tested the breaks and it does stop the car when fully depressed to the floor and the pedal comes back up but is very soft and weak.

Also the brake light stays on the dash.

I checked the paper work that came with it and it mentions that if this occurs you remove the master cylinder and adjust the push rod

In or out depending on what’s required then re fit the master cylinder.

As it came already built I would have thought it would be ready set. And this seems a bit daft for setting breaks having to keep taking the master on and off Until correct especially as it means keeping it steady to stop break fluid spills.

I have changed pads, lines etc on many cars for years but this has me baffled
Any ideas as to anything else it may be.

Getting desperate.