Repeat after me, "Pressure Regulator" -- "Pressure Regulator"
That's right, this little beast is a pressure regulator, "NOT" a proportioning valve. It proportions "NADA". It is the first anti-lock brake device ever to be installed on a Porsche. Here's the story:
The 914 is a mid-engine automobile. As such, it has amazing handling characteristics... to a point. Once that point is reached, it can bite you like a rabid dog. We're all familiar with the child's play toy the "top". It's a physics principle called "Polar Moment Inertia".
Porsche knew the issues with rear and mid-engine automobiles so, they installed this pressure regulator in the system to prevent the rear wheels from ever locking up. You see, if you lock your rear wheels in a rear or mid-engine car and your front wheels are anything but straight, you can be that "top" we were talking about and, that's not a fun toy at that time. This pressure regulator senses a panic stop (which it is set and locked at 525psi). Once it sees that pressure on the system, the valve body slides over and activates a microswitch which shuts off pressure to the rear wheels. This is a panic stop mind you, this device flows fluid like a "T" fitting until you need it. That's why we insist that it proportions "Nothing". Once the pressure equalizes in the chamber, it brings the rear calipers back into the circuit. This is "split second" stuff gang, not a leisurely event.
So, yeah, the internet guru's steered you in the wrong direction. You "never" want to replace this regulator in a street car and, you want to make sure it's operating properly otherwise, your rear calipers aren't even working and, that's 30% of your braking power.
Here's what we do; We will completely disassemble the regulator. We will zinc plate the spring housing and the valve housing. We will clean and reseal the valve body using new EPDM seals throughout. We will repaint the freshly plated spring housing and the valve housing. We will zinc plate the mount and the fasteners. We will then reassemble the regulator and reset the pressure switch to 525psi. We will then epoxy the lock nut just like the factory and send you back a perfectly operating pressure regulator that not only makes sure you and your passengers are safe but, it give you back your full braking potential so your 914 stops as good as it did when it was new (and that's still damn good my friends).