[cparman]

Please tell us what the brake setup is on your truck. I assume that it is disc/drum manual brake? As amazing as it might seem in today's world, GM really did have some GREAT people back in the day. Like smashingchuck said the line from the prop valve to the rear brakes really should be 1/4". I have done disc conversions, and gone both ways 3/16 or 1/4. If you are having a problem, I would bet the problem will be still there AFTER you work your fanny off to install a different prop valve. Brake chatter/vibration when brakes are applied usually is a drum or disc that is out of round. If I were you, I would drive my truck on a non busy street, and at about 35-45 apply the emergency brake. NOT THE FOOT BRAKE! Pay close attention, and see if you feel this chatter/vibration. I would recommend not leaving the brake on for long. 10-20 seconds, and then release it. If the vibration is there, have your drums check for minimum thickness, and then have them turned. Tell the shop who turns them what you are up to, and see if they find them out of round. IF there is no vibration found when the E BRAKE is applied, then you have a front drum/rotor that is out of round and do the same thing. The PROP valves on our stuff fail because of a leak, or being tripped, and the owner/shop not knowing how to center them. In respect to ANY prop valve, you can use one 3/16 and T it to both front wheels or use both.
Chip