Don't forget that suction pressure may vary with engine bay temperature. If the motor and engine bay temperature is hot then the suction pressure may be as high as 40-50 psi. This is assuming that the system is R134a.
When people say "evaporator" I'm assuming they mean "accumulator", the big silver canister? The evaporator is up in the firewall.
If the accumulator isn't getting cold then the system is either low on R134a or the compressor's efficiency has dropped off. The high side pressure should be roughly 5*low side pressure. If the high side pressure is >175psi when the low side is 35 psi then this may be an indication that it's just low on R134a. Conversely, much lower than 175psi then compressor may be damaged.
