Brake pedal sinking to the floor

Hi All,

I am new to this forum.

I have a 2005 citroen berlingo which I have had for a few months with 110k miles.

Not too long ago the Stop light along with the hand brake light on my dash would flash on and off every now and again when i would drive my car from cold start.

I recently changed my front brakes and had a new MOT with no issues.

However for the past 2 weeks the Stop light and handbrake light now remain on at all times with a intermittent beep sound, and i have noticed when i apply my brakes the pedal sinks to the floor which takes a while for my car to come to a stop, making emergency braking impossible.
When i pump the pedal and press the pedal down it has very slight pressure and then sinks straight down.

I have done the following:
- Checked brake fluid which was fine but topped again
- Checked for leaks but nothing found.
- Checked rear brake shoes which are in very good condition
- Pumped the brake pedal to check for any air lock in the braking system (The driver side rear was fine, however noticed no brake fluid was coming through the passenger rear. Eventually some oil came through very slowly but this had no effect on the level of the brake fluid by the engine so assuming the oil was whatever was already there.

Please can someone advice me on what the problem could be?

Many thanks