My 'Cuda wants to lock up and slide way too easy....

In the rare event of a hard braking situation, my 68 Barracuda wants to lock the wheels and slide.....
Under normal conditions the brakes are beautiful. .....
Non-power brakes...front stock disk with rear drums...
I'm not "standing on the brakes" either but just pressing harder.
Been driving for 35 years so it's not like I don't know how to use brakes....
My only thought is proportioning valve???

Jeff