2006 Passat brakes

[ch427]

Brother in laws car has an issue where the brake pedal becomes spongy over a period of weeks, I believe the fluid has been flushed twice by local garages with no success. I don't think there are any visible leaks.
Are there any common faults to look out for on these?
I will be getting the full story later tonight so will update accordingly but just wondered if anyone knew of common faults.
Car is a 2006 2.0 tdi estate.
Thanks in advance.

[ch427]

Just found out from him a faulty rear caliper has been diagnosed so i can close this off now.