Power steering pump pressure relief valve questions

[Chris Wilson]

Toyota road car PAS pump. They give "issues" and I suspect the conical brass gauze filter in the compression spring end of the valve shown gets blocked. The factory manual gives no detailed description of how the PRV works internally. It's in the sliding vane rotary pump casting itself, which uses a remote fluid reservoir. Failure mode is zero power assist, and zero fluid movement in the remote reservoir, which also houses the return hose stub from the steering rack. So I assume the valve is allowing fluid to recirculate within the pump's casting in such a way no swirl is evident in the fluid reservoir mounted 6 inches above the pump itself on a rubber hose.

Can anyone say why the valve internally looks like it may be two piece and how the 2 ball check valves might work please? The best illustration of the valve I can find is blown up at http://www.gatesgarth.com/valve.jpg The left hand side of the valve is against the compression spring for it inside the pump housing. the right hand side forms the convex seal for the outlet female port that serves the pressure side of the steering rack. the rack return goes into the remote reservoir. The holes that they show having compressed air going in to them as a test?? Not sure where they feed or are fed from. Could be pressure side or return to the inlet side of the pump rotor... Many thanks.

[powerstroke]

I would guess it works with simple spring loaded balls so if you hold the steering against full lock or something gets blocked the excess pressure forces the ball of its seat allowing the oil to recirculate maybe the spring is broken so the ball isn't being held against its seat??

[GreenV8S]

I haven't figured out how the internals of that pump work, but I guess the basic principle is similar to an oil pump pressure limiter where oil pressure at the pump outlet forces a valve open against a spring to allow excess fluid to bleed back to the inlet. It isn't exactly constant pressure but tends to reduce pressure changes with revs, viscosity etc. A common failure mode with these is for the valve (ball, piston etc) to jam in the open position. Maybe you're looking for something similar here. I guess a broken spring would cause similar symptoms.