Okay, the valve on the frame is the Proportioning valve and mine is not working well and is leaking also. There are plenty of spare(i.e., used) parts in this neck of the woods, but like you, I don't trust used parts when it comes to the brakes(bad brakes could kill someone). The valve is easy to replace. There are only five lines to loosen and two bolts that hold it to the bracket. The problem comes in when bleeding. The bottom of the valve also needs to be bled(the spring portion at the bottom of the valve doesn't bleed well). It takes a 3/4 box end wrench to loosen.
You MUST have clear new fluid flowing solid(no bubbles) flowing from it in order to ensure that the valve has bled properly. Then, bled the brakes normally.

A footnote: if the new master cylinder has not been bench bled properly it will not work(spongy pedal, soft brakes) properly.