Rear Disc cracks

[Dom Knight]

Lately I have noticed what appears to be cracks in the rear disc's.
The lines seem to run from the center of the disc to outer edge.
How can I be sure they are hairline cracks ?
The one on the passenger side has an obvious looking crack.

 

[Dom Knight]

I replaced the rear disc's today but found I had to screw the rear pistons in a bit to enable me to get the caliper back over the pads.
I didnt need to change them as they were nearly new.
But I had to remove the calipers to enable me to screw the piston in on each side.
So this meant bleeding the rear brakes again.
I thought everything was okay but when I started the car up the red brake warning alarm and symbol was displayed on the dash.
Also the pedal pumps up and then loses pressure and then firms up and loses pressue once again.

Does this mean I need to bleed the brakes again ?

 

[s3dave]

Yes Bleed them again, the red warning is low fluid in the reservoir, so make sure thats topped up

 

[Dom Knight]

Well I bled them over and over again with the same result.
However I was thinking what if I bled the master cylinder.
So I bled both nipples and the brake pedal firmed up straight away with lots of air coming out of both.
Lovely brakes now.