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I have been a professional master mechanic for over 35 years and am happy to help you with this.
There are 2-3 basic systems involved here. you have the mechanical (frame assem) and the hydraulics and a switch. I have worked on a lot of these GM Olds converts along with the 442 cutlass and if its not the actual linkage binding at a certain point, then its the seals inside the hydro pump that are what we call "by passing" meaning that the seals are just worn and when pressure comes up against them they leak(internal bypass) instead of maintaining the hi pressure needed.
Bot***** *****ne is i suggest (if havnt done) is to disconnect the pump from the frame of the top, and just manually move it forward and back to check for smooth operation. WD-40 the hell out of the hinge points.
Then take the pump out and have it pressure tested. It's probably gonna take a new reseal kit.
Nice car and if you have any further questions just let me know.
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