I had the same thing happen when the fluid level got low in the resevoir. After I topped up the fluid, everytime I turned the wheel, I blew fluid out the vents.
Sounds to me like air in the system. Let the car rest a bit, top off the fluid then you need to bleed the system. There is a bleeder screw on the top of the steering box, crack it open, put a piece of tight fitting rubber hose on, run it to the resevoir and start the engine, but DO NOT turn the wheel. Let it idle until the fluid run clear of any air.
Or, you can top up, start the engine and VERY slowly turn the wheel stop to stop. One direction will cause the level to drop, the opposite direction will raise the level. Once the fluid is clear of air bubbles, the system is purged and should no longer blow out the vents.
Next you need to find out where you are losing fluid. Front pump seal, steering box, low pressure return line??? If it's the return line I have an easy fix for it posted in the Vintage forum titled "handy little tip", try a serach. Keep an eye on the fluid level and keep it topped up, find and fix your leak.
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