The "pressure relief valve" is actually the "residual pressure check valve" I cited earlier in this thread.
If the master cylinder has one, it will require disassembly of the brake master cylinder to see it.
These valves generally are the very last component to come out of the master cylinder during disassembly and may in fact be be an integral part of the piston group of parts.
The residual pressure check valve has some type of spring to push the valve face down against the fluid outlet/return passage of the cylinder.
It is this spring which sets the amount of the residual pressure which will be held in the plumbing between the brake master cylinder and the wheel cylinder rubber cups.