On demand hydraulic pumps?

'On demand': switch it on when it's needed; switch it off when it isn't.

30gpm. OK. What discharge pressure?

You may be looking for a load sense pump. For information on LS pump you may want to try:

http://www.hydraulicsupermarket.com/

For a pump and a few schematics:

http://www.eatonhydraulics.com/products/pdfs/E-PUPI-TM002-E.pdf

Good luck.

Sounds like a closed centre hydraulic system with a variable displacement pump that runs all the time, but only pumps out fluid when there is a demand. The demand triggers the variable displacement mechanism (usuallyu a swashplate or the like) that puts the pump "into stroke" and causes it to pump. This is pretty standard in the industry and any one of the major hydraulic suppliers: Rexroth, Vickers, Eaton, etc. can help there.

Replacing the two stage for a single stage may not work with out replacing the motor. The two stage pump will have dual gpm ratings one for each stage of the pump.

At high pressure the motor may not be able to handle the load.

<luke>