What would be the HPLC system normal backpressure when mobile phase is pumped through? [system is disconnected from guard col., main col., and detector]

It would depend on the mobil phase, flow rate, tubing diameters, total tubing lengths. What is it now?

I have a quaternary high pressure pump mixing 50% methanol, 40% water and 10% buffer @ 1 ml/L. The pressure was 80 psi in the beginning but is now up to 150 psi. I have not changed the individual solvent filters inside the bottles for over a year. The tubings are standard for perkin elmer.

The solvent inlet filters will not cause increased pressure. Do you have an in-line filter either between pumps and injector or between injector and column? One of these may be clogged. I have also seen high back pressure caused by an old piece of tubing. Start removing tubing starting at the outlet and working your way back to the injector then the pump, until you see a drop of pressure, then replace that piece of tubing.