I have just replaced the brake fluid and have bled the system through the screws at the four calipers (RR first, then RL, FR, FL). Unfortunately, there still seems to be air in the system: when braking, the pedal now goes all the way down the first time with no braking effect, the second time I can feel the brake pads engaging, and the third time the car slowly stops (with the pedal going all the way down).

Is the master cylinder on a 1979 240D bleedable at all? My FSM says (for the 240D):
"To remove all air bubbles from tandem main cylinder, step down fully on brake pedal three times while bleeding with bleed screws of brake pedal opened."

However, I cannot see any bleed screws on either the master cylinder or the brake pedal. Can someone chime in and help me out on this one?

__________________
1979 240D, W123, 105K miles, stick, white w/ tan interior.