Is the return hose in good shape? Is the gasket on the reservior cap sealing?
I followed this procedure for flushing and it solved the foaming problem:
Flush Power Steering Fluid Exchange
You'll need a helper. The steps are rather simple.
1. Lift front tires off the ground.
2. Disconnect the ps return line at reservoir, drain ps reservoir.
3. Plug open connection at the reservoir.
4. Secure the return line to a generously-sized container (at least one gallon). If needed, add a length of hose.
5. Fill reservoir with new fluid.
6. Line up your bottles of new fluid for quick pouring.
Here comes the fun part...
7. Have helper turn car on, and immediately have him go through full range of steering motion (left and right) all the while you are trying to keep the reservoir filled. Shut off car after about 3 quarts are run thru the system.
8. RE-CONNECT RETURN LINE.
9. Clean up mess.
10. Top off fluid.
11. SLIGHTLY CRACK BLEEDER FITTING ON TOP OF BOX WITH CAR RUNNING AND HELPER TURNING WHEELS UNTIL ALL AIR IS EXPELLED AND FLUID STARTS TO COME OUT.
The front tires must be off the ground so the system is unloaded, otherwise a lot of fluid is forcefully pumped out. The return line must be secured to prevent a big mess. Keep at least a couple quarts of ps fluid on hand. The helper must know what they are doing.
1972 280SE 4.5