When your front calipers drag on the rotors going down the freeway the heat generated will boil the fluid in both circuits causing total loss of hydraulic pressure. You'd rear-end someone at the toll booth if it wasn't a free way. Next high speed run you make, check the temp at the wheels. You most likely need a new set of front calipers.

Slow sinking of the pedal while at a light is what happens when the master cylinder is scored. It bypasses fluid. Perhaps someone before you bled the brakes with the pedal before you did with a pressure bleeder. Get a new master.
--
Art Benstein near Baltimore

From Rock Auto's newsletter:

I had been planning on replacing all of the brake lines on my Chevy Silverado for some time, but never got around to doing it. But after a weekend of hauling
loads of wood, one of the rear brake lines blew out and forced me to take care of the delayed repair. Since I would be doing all the work anyway, I also decided to replace the front calipers.

I was rushing through the job and not paying much attention. Once I routed all the new lines and put on the new calipers, it was time to bleed everything. But
during the bleeding procedure, I noticed I had a ton of fluid pressure in the rear calipers and almost none in the front. Also, the pedal was not getting firm.

After about an hour of scratching my head and much colorful language, I discovered that I had routed the brake lines through the proportioning valve incorrectly. But, my story does not end there.

I rerouted the brake lines to the valve and tried bleeding the brakes again. My father and I bled the brakes over and over and over, but the pedal was just not
getting firm. I had bled brakes before, but never have I had to bleed them as much as I was. I looked on forums, asked friends who had similar trucks and looked at my brake line routing again; nothing worked. I eventually surmised that it might be the ABS module needing an electronic scan tool to properly bleed. I did not have that type of tool, so I threw in the towel and took the truck to a nearby shop.

The next day I got a call from the mechanic...I could not believe what I had done. I had put the calipers on upside down and on the wrong sides! With the bleeder screws at the bottom of the calipers, no amount of bleeding would have removed the air bubbles from the brake fluid. I had to pay the mechanic $240 to fix my blunder! Do not make the same mistake that I did. Take your time and do not rush a repair.