The present invention relates generally to a power-assist device for an automotive steering system.  More specifically, the invention relates to a power-assist device employing a spool valve-type pressure control valve. 
Hydraulic power-assist devices employing unbalanced cylinder devices have been known to be of simple structure and low in cost.  In such hydraulic power-assist devices, a piston firmly mounted on a piston rod is inserted into a hollow cylinder.  The piston defines two pressure chambers within the hollow cylinder.  The piston rod extends through one of the two pressure chambers, resulting in a difference between the effective areas of the piston subject to fluid pressure in the two pressure chambers.  In particular, the pressure chamber receiving the piston rod will have a smaller effective area than the other chamber.  The former pressure chamber will be referred to hereafter as "smaller area chamber" and the latter chamber will be referred to hereafter as "larger area chamber". 
Due to this difference in the effective areas, when fluid pressure is applied equally to the smaller and larger area chambers, the hydraulic forces applied to the two sides of the piston to assist steering differ.  As a result, the piston is displaced into the smaller area chamber until the forces applied to either side of the piston balance. 
There are various approaches to solving this problem of the unbalanced cylinder device.  One typical approach is disclosed in German Patent First Publication 29 31 139, published on Feb. 5, 1981.  The disclosed device uses a pressure control valve employing spool valves to adjust the supply of working fluid to the smaller and larger diameter chambers.  In the shown device, a spool valve controlling the supply of working fluid to the smaller area chamber has a relatively wide annular groove for relatively small throttling resistance to fluid flow.  On the other hand, the other spool valve controlling the supply of working fluid to the larger area chamber has a relatively narrow annular groove applying a greater throttling resistance to fluid flow.  Thus, the increasing rate of fluid pressure into the smaller area chamber is greater than that into the larger area chamber so that a higher fluid pressure is built up in the smaller diameter chamber.  This pressure difference balances the forces exerted on both sides of the piston so as to eliminate any bias relative to the neutral position thereof of the steering wheel.  This balance also balances the steering load both steering directions to ensure a good operating feeling. 
However, this conventional pressure control device tends to be mis-assembled due to the fact that the spool valves have essentially the same external diameter.  Since the two spool valves have the same external diameter and are of generally the same size and shape except for the width of the groove, workers installing the spool valves in valve housings must pay careful attention to avoid mis-assembly.  This obviously lowers efficiency.  Furthermore, some instances of misassembly are inevitable.  When the spool valves are installed in the valve housing backwards, the piston will generate a strong steering bias and so subject the vehicle occupants to serious danger.