68 power drums - ERRATIC

[AnotherWs6]

Hey there, I have a 4 wheel power drum setup on my 68 burb from the factory. Previous owner installed a new master cylcinder and booster from a local auto parts store type place. They SAY they are correct but who knows.

The issue is this.... in low speed situatiosn, parking lots etc, the brakes are usually super touchy and will lockup if you look at them the wrong way. Once at speed the brakes require a lot of travel before they even begin to bite. They have a lot of stopping power once you get through the dead zone, but hit them hard and they occasionally make the truck darty and chirp/lock up/ The brakes themselves were all gone through. I installed a new check valve in the booster and it seemed to help a little. What do I have going on here? Wrong parts? Vacuum issue? It makes driving this thing a little scary@