I've swapped out my orginal Drum braked axle from my GD1 sport with a Disc axle from a GD5 and now the brakes are disc all round

great
However now I get spongy brakes at first and then the handbrake light comes on permanently and the brakes then work as normal.
Any ideas as to what might be causing this I'm currently running my brakes without the ABS sensors on the front end as this had greatly improved my braking abilities as well as having black diamond discs and pads on the front.
Cheers
Andrew