| Have you changed any lines recently?
Used new copper washers if you have?
Are you losing fluid level in the master or is it just "going soft".
Any leaks will be apparent at the washer joints cos it's under pressure.
If it's going soft cos of air, then is your master level staying high enough when the lever operates and is your master upright or have you put it at an angle on new bars?
Check yer nipples |