I do not understand the need to stand on the brakes. You need to check the wheel cyl, brake shoes and check to see if the drums are too worn. I have only 1 Stude with factory power disk brakes out of 9 cars including a 62 GT w/o PBs and have NEVER had to stand on them. I never saw Grandpa having to do that either and he stopped driving the GT at age 89. Thoroughly check ALL components of you brakes. I will look at a friends '65 tomorrow and tell you what chebby he got his power brake system from. Seems like it was about a '68 or '69