I recently worked on my '74 to improve the heat output. I changed the cables
 on both sides, but in retrospect I think that new cables may have been 
 unecessary. 

 On my bus, pulling the red lever all the way to the end of it's travel was
 not opening the flapper valves all the way. Based on the improvement in 
 heat output I got with my repairs, I'd say that these valves are NOT like
 waterfaucets where you often get 95% percent of the flow from half the 
 travel of the valve. 

 After threading the cables down their tubes and attaching the front <lever>
 ends, I set the lever in the full on position. Under the car, I pulled the
 flapper arms to the fully open position and then tightened the cable clamps.
 This is kind of a three handed job and it's not as easy as clamping the 
 cables when the valves are in the closed position, but in that circumstance
 some of the travel of the lever up front is wasted taking up the slack in
 the cables.

 In addition, cleaning and oiling the pivots of the flapper valves seems to
 have made them a little more cooperative. Since I changed the cables at the
 same time it's hard to tell which contributed the most, but the red lever
 now moves without a lot of resistance and the heat output of the system 
 has increased dramatically. For the first time in years, passengers have 
 asked me to turn the heat DOWN, because they were getting too hot. Okay,
 okay, I know, it's California and it was only 40 degrees F outside......

 Dave Kautz