The two vacuum lines at the p.s. pump are suppose to idle the engine up slightly while turning and stopped, if those are not connected, or the valve on the pump is bad, it will still turn, just a little harder at idle. It sounds like your problem is bigger.
I'm thinking it could be the pressure relief valve on the pump, but usually if that sicks, you get no power steering, or the speed sensitive steering valve on the rack. That could be stuck in the high speed setting which lowers your power assist.
Check this if you have Progressive Power Steering:
MK3 TSRM On-Line
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'88 Supra Targa 5 speed w/ '90 7M-GTE 
Currently Running Good  
For Sale: Light blue body parts, Blue interior parts all from '90 supra targa turbo manual.
Also for sale: R154 trans, brake pedal, master cylinder, booster, abs actuator, sensors, abs ecu, driveshaft, 3.90 lsd rear end, radiator, wiring, speedo cable, 8 saw blade wheels, '88 ecu, relays, cruise control actuator (bad cable), empty fuel tank, etc....
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