Rear brake woes... air in the system AGAIN?!

I rebuilt all the calipers, flushed/bled the system once. It felt okay, but got really spongy over a few weeks... full pedal travel and nothing happens spongy. Figured I didn't bleed the system properly.

So, I bought speed bleeders and thoroughly re-bled the system as per the service manual... making sure I followed the exact sequence including removing the front left caliper and orienting the slave master cylinder at the right angle. A ton of air came out of the PCV. Pedal felt fantastic immediately afterwards, braking performance was improved, and I was happy as a clam.

That was about a month ago. Spent the last couple weeks replacing all the coolant lines and got her back together for a test ride.

Rear pedal is spongy again!!! What gives?

I'm suspecting a leak someplace where air is getting into the system, but there is no loss of fluid at the rear reservoir nor any wet spots.

Any suggestions?

Sounds very similar to an issue I had with a car of mine. I ended up replacing the master cylinder as it had failed.

If fluid is bypassing the internal O-rings in the master it will give that feeling of sponginess. 

I have also seen cases in the car world where the flex lines are ballooning out