if your fluid is low theres only two possiblities (depending). one, is that you have a leak somewhere in the system.you only have to add fluid if theres a leak. two, your brake pads are low on friction material and that causes the pistons in the calipers to be extended out farther causing less fluid in the reservoir, but if this is the case the fluid level wont be too low. more than likely number one is the culprit, so id say find the leak and go from there. and no, bleeding the system and putting fresh fluid in wont change anything., hope this helps and good luck.