C-10 brake pedal problems

[Mr Twister]

Hi
I have a 1972 C-10 4x4 brake problem I need some help with please. The break pedal falls to the floor about I out of 15 times I use the breaks. If I pump it a few times it comes back up. I installed a new, not rebuilt, master cylinder and bled the lines. Much better pedal now but stills falls to the floor occasionally. I don't see any line leaks. Could the proportioning valve be causing this?
This is not a show truck. I've had it about 35 years and three motors later it's still on the job.
Any advice would be appreciated.
Thanks!