Correct me if I'm wrong, but is this a PD pump, rather than centrifugal? If it's the type I'm thinking of, the pressure will build up and up until one of the following happens:
1 A relief valve lifts.
2 Something - e.g. a flange or pump internals give way.
3 The back slippage through the pump equals the forward motion.
4 The motor is no longer able to produce the required energy and so trips on high amps.
The max. pressure may simply be the design pressure of the pump. Speeding it up will not help - it will only affect the flowrate (Unless the back slippage is the issue).
On the other hand, if it's a centrifugal pump, you can use the pump affinity relationships.
As TD2K stated - we need more details to help.