I agree with beltfed. I had a '70 Monte Carlo that had that problem sporadically. I was told by an old mechanic to change both front rubber lines as people tend to snap vice grips on them when doing brake jobs to try and save having to bleed the brakes afterwards. This can cause the inner liner to crack and flop down when pressure is applied and then blocks the fluid from releasing when you release the pedal. Once I did that I didn't have any more issues.
