Looking to buy a rear brake proportion regulator from a 914-6. Part number is 91435506700.

Contact Eric Shea at PMB Performance. They can refurbish (restore) your existing valve.

PMB did mine.

Even if you find a used one, you'll still want to get it restored (assuming you don't already have a core). Rear proportioning is a critical part of the brake system and you want to have it working properly.

Eric has done a lot of great work for me. Problem is I do not have the part and need to source it before restoration work.

I believe a stock 4 valve can be adjusted to the 6 specs. They look identical on the outside.