Datsun 2000 Brakes

I need help!!
I have a 69 2000 Roadster. After I drive it a while, the front calipers lock and the pedal stays hard at the top. After sitting a couple of minutes they free but then lock up again after a few applications of the brake. I put new master cylinder, front calipers, rear cylinders and pads all around and again....same problem. ANy suggestions???

There's not much left except the proportioning valve. or you could have a pinched line. The pedal pressure would push the fluid through quickly and then take awhile to flow back to the resevoir. Do both calipers lock up or just one?

Thanks Jon for the reply. I replaced every part of the system except the 3 rubber hoses on the front. Still had the problem, Finally replaced the 3 hoses and VOILA!!! no problem. Sort of an expensive lesson. IT seems the hoses were old thus collapsing when the pedal was released.

Well, after replacing everything, I am going to be set for a long time on brakes.