It sounds like one of the seals in your MC is leaking. If you had air in your system it would be spongy all the time. If the seal on one channel is bad the pedal feels like it goes to the floor but the brakes still work (on the other channel). Does your brake warning light come on? The balance valve should trigger the light if there is a bad seal. You can change the MC without bleeding the system. I have done it. As you remove the lines tip them up slightly and the fluid will not drip out. Then I filled and removed the air in the rebuilt MC before I attached the lines to it. with the resevoir filled the MC drips brake fluid so put something under it to keep the fluid off the paint. then just attach the brake lines. As you are tightening the lines any air in the joint bleeds out because of the fluid dripping out of the MC. I don't know whether it always works, but it has always worked for me. I did it on my 75 245 and many times on the clutch master cylinder on my Saab 900. I hate bleeding brakes so I avoid it whenever possible. I also find you have much less waste brake fluid to dispose of using this method.