My car has a remanufactured P/S pump with new hoses. Steering box is original from a '84 Z-28.
When the engine is idling, there is no power steering, but as soon as I bring the RPM up, it's working.
What would cause this?...

 

sounds like you git a bad pump....try another one... remanufactured pumps are hit or miss

 

Generally this occurs after the Pump has worn enough that the Pressure produced is too low at Engine Idle.

However there are other possibilities...
Metal-Debris can create a restriction or clog.

If the Pump has just been installed, and has always performed this way...
The Flow Control Fitting may have an Orifice that is smaller than the previous Pump used.
The Image below shows the Flow Control Fitting:


The Image below is of the back of the Pump.
The Outside of the Flow Control Fitting is seen with the letter "A" marked next to it:



The next Image below shows the Orifice that is in the center of the Flow Control Fitting:



Warning, when removing the Flow Control Fitting, the Pressure Regulator Piston and Spring can fly out of the Bore.
Shown below:



The Hyper-Link below contains a basic Article that goes over this information...
If you would like to have a greater understanding of how things work, feel free to ask:
LINK

 

After my original post, I did a search, and realized that I never did a proper bleed of the system when I installed the pump. So if that doesn't fix it, I'll be replacing the pump (again!). 'Only reason I replaced the previous one is because some idiot in the past damaged the case with a prying tool and caused a major leak...

 

There's really no special bleeding procedure...

Turn the wheel almost lock to lock. The flow of fluid will flush out any air. The pump is constant displacement; the fluid is constantly circulating through the lines and gearbox; all you have to do as the installer is move all the parts around in the box so that all the little pockets where it could hide, get flushed.

The spool valve has a return path in each side, such that if there's no torque on it, fluid flows freely though both, back to the suction side of the pump. To control the assist, the spool valve closes off the return to the pump on the side it needs to push toward, building up pressure in that direction, until the added force from the power assist opens the valve back up, thereby balancing the assist against the driver's effort. When you turn the wheel to full lock, the pressure can go extremely high, because the spool valve stays closed, and the assist can't move the gear any farther to relieve the driver's effort on the wheel. That's what the relief valve is for; it limits the pressure to 1200 or 1500 psi or so, to keep the pump from getting hydrolocked, or the pressure line exploding, etc.

I tend to agree with the bad pump diagnosis. About the only possible "explanation" that fits poor low-speed assist is, the pump isn't moving as much fluid as it should.

 

loose belt?

 

Nope, belt is tight, and not one bit of noise from the pump...

 

Did you take a look at the Information, and the Hyperlink that I Posted?

Also, I have Pumps that I Modify and rebuild for Hydro-Boost Power Brake Booster Systems.
If you are going to swap out the Vacuum Power Brake Booster...
I have complete Sets of Pumps and Boosters.

 

This pump might be under warranty. If bleeding the system doesn't help, I'll be replacing it one way or another...

 

in any regard, these pumps really are pretty simple, they either work, or they dont