Re: 94 Marada Mercruiser Power Steering Issue
Did you keep the old fitting from the original return line and put it on the new pump? Perhaps there's some shrapnel in the system that is stopping in that line end. Maybe the fitting is blocked or crushed. Back flush and clear it. If it's questionable just get a fresh one.
It's also possible there is more c___ coming through from the rest of the system. Could have been in there from before.
I'd take apart the lines and blow them out, and as was said, back flush the cooler.
Also, you can try to back flush the power assist unit. It might not cooperate though because it has small orifices on it's inside.
To do that, wiggle the control valve actuator while blowing air or forcing fluid through there backwards. I wouldn't rush into that though. It sounds more like your blockage is at the inlet to the pump, or there's too much pressure in there.
The new pump would have a built in pressure regulator as part of it. So it's not likely you had two in a row with the same failure.
Here's a thought. The return pressure comes up with demand. And the demand happens when the control actuator is off it's center stop point. If the cable is bound up and the control valve is always pushed to one direction or the other the pump sends full pressure through for too much of the time. Normally the pressure isn't full, it only goes to full pressure momentarily while the wheel turns.
Does it sound as if the pump is laboring all the time? Does the steering wheel turn to it's stop when you start the motor? If so you could need to balance the power steering or free up the actuator pivot so it can center easily. To balance it there's a nut on the starboard end of the valve body under a little metal cap, it's behind the big cylinder. You just turn the nut a little to adjust the side to side balance so it stays in place with the engine running. If you have to fight the wheel underway you might have that issue.
Just watch out for the power steering arm when you go to adjust that thing, it can injure you. Make sure you can keep yourself away from the machinery.
I'll try to pull out the pages from my manual so you can see what I mean. I wish posting pdf's didn't take so many steps.