78er Chevy C10 Short Bed, brake back makes too fast

Hi, I have a problem with the brake. All brake lines, hoses, BKV are new. On the brake test stand everything looks good, but when you drive, the rear wheels block in front of the front. It is empty simply too light in the back. My mechanic said that there are brake pressure regulators with fixed values, which are dragged into the line to the rear. But he can now recommend me no one with a certain value. So would be a try and error. Since here also a few of you such a can you help me with what kind of pressure reducer I’m supposed to put in? Or can it be that I’m missing a weight plate (the old Econolines also have in them)? Greetings Chris