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Power Steering uneven pressure

I seem to lose power steering pressure when I hold the wheel in a turn. It is only temporary as it cycles until I release the turn. I happens turning in both directions and a any speed. I have replaced the struts and strut mounts and have rotated the tires. The power steering fluid quantity is good with no leaks.
1995 Buick Lesabre.

Sounds like the drive belt might be slipping.

The problem might be with the spool valve O-rings/bore in the rack and pinion assembly. GM vehicles of this vintage were notorious for this problem. The O-rings of the spool valve would wear grooves in the bore of the aluminum casting of the rack and pinion assembly. Then when the steering wheel is turned the hydraulic pressure which is supposed to provide steering assist would by-pass the O-rings/bore and and you lose power steering.

If it is the rack and pinion assembly causing the problem, the good news is a remanufactured rack and pinion assembly will have a stainless steel sleeve where the O-rings seal instead of the O-rings trying to seal to the aluminum casting bore.