Brake problems on 92 silverado

[WS6]

Ok I know this isn't really the proper forum, but I am hoping I can get some help anyway. 92 Siverado standard cab, 2 wheel drive, 350 V8 high mileage. The truck will bounce when braking and your at 35-45 mph. It does it in this range even if you start above 45 mph and go below 35. I've never seen anything like this. It feels like bad wheel hop honestly. Brakes are new all the way around except the front calipers. They are not leaking either. New shocks all the way around. I thought the old worn out shocks may have been the culprit, but that did nothing. This did not start happening with the new brakes. The brakes were replaced and worked fine and then a few thousand miles later this started happening. Nothing is out of balance and the tires are in good shape. Rear drums and pads don't show any odd wear. My thoughts are towards the master cylinder. It has always felt soft to me but the brakes worked perfectly fine even under hard braking they worked great. I thought maybe Silverados have soft pedals. This isn't my truck and I am not fimiliar with how they are supposed to feel. So I am lost at this point. If you have any ideas for my to test out or any thoughts on what may be the problem I am all ears. Thanks.

*****UPDATE*****
Ok we finally gave up and took the truck into the dealer. The pedal had gotten very soft and would go to the floor with constant pressure. The bouncing was getting worse and happening beyond the 35-45 mph range as well. We tried replacing the master cyclinder and that did nothing. Actually it got worse. The pedal was the same whether a bleed screw was open or not. We firgured there was air trapped in the system some how. I bench bled the MC before putting it on the truck and line bled the system for quite a while. Never made a difference.



So the dealership pressure bled the system and turned the rear drums. No more problems. We will now be purchasing a pressure bleeder. I haven't had any issues with manual bleeding even one person bleeding using a half filed jar with the tube running into it. These methods always worked for me in the past without issue. The drums even though they were brand new needed turning appearently. I wish I had a caliper big enough to measure drums. I didn't notice anything with a visual inspection. I guess I missed something. Anyway it is fixed and now everyone knows the cure incase this crap happens to you.

I still don't see how the pedal became so soft when the bouncing was the first thing to happen. As it continued the pedal became soft. Could the out of round drums have cause the wheel cylinders to push in and out quickly somehow building up air that way?