I had a similar issue with my Plus4 of a similar vintage. I was never happy with the feel when I took ownership of the car, which was particularly bad when going down a hill with a T-junction at the bottom where the pedal went to the floor.
When I purchased the car it had been serviced every year by the same main dealership and had the benefit to check all the invoices and found the car had never had its brake fluid changed. I changed the fluid and checked the rear cylinders which had recently been replaced, however still not happy with the brakes.
In the end I decided to replace the brake master cylinder only to find the car transformed!!! I was grasping at straws as the car had only done 10,000 miles! But problem solved in my case.
Hope this helps, also, the master cylinder on my car was based on a Mini so cheap to buy. If you look on the body of the metal brake cylinder you will see the manufacturer’s part number.
