I just finished putting a disc/posi/3.73 rear in my '86, along with the correct master cylinder, proportioning valve, and parking brake cables.

Well, today I was "exercising the posi" in my alley and when I hit the brakes at the end, (probably going 30mph) the rear wheels started violently hopping! It almost felt like some kind of extreme ABS. This had never happened with my old drum rear.

Has anyone else had this happen?

is the porportioning valve all the way open? or is it non adjustable if thats thecase the lines may be shut off look for a button type thing on it somwhere to be pushed in

Its a brand new factory GM part. The brakes bled fine, so I'd assume they were all getting pressure. That was the only time it ever happened. I'm thinking it may have had something to do with the rear tires being sticky from burning out, and then the sudden moderate brake pressure.

Who knows.

...if the rears locked before the fronts, he prop valve is wrong. Sticky tires or othewise.
- also, how good are your shocks? and are you sure everything is tight and in the right place?

Sounds like what we 3rd gen road racers found out quickly, too much brake in the back leads to wheel hop under heavy braking. Dial out some of the rear or go to less grippy rear pads.