Weak pedal even with brake swap

[bad86ss]

86 Monte Carlo SS
My cars brakes have always been weak. Just finished c5 fronts, ls1 rear disc and the pedal still goes nearly to the floor. Haven't touched the proportion valve, running stock combo valve. That shouldn't make the pedal suck though. It's like there's air in the lines, but its been very well bled. Does this sound like a master cylinder gone bad? What's a good donor car to order one off of? From what I've read, 1" bore should give a better pedal feel

[Z06vet]

I have used a mid 70s corvette master w 1" bore with my C5 brakes. I have good pedal but not so much assist that they lock up too easily. If you are 100% sure there is no air left in the lines & no leaks, it could be the master. If you pump the pedal quickly, does it come back?

[latamud]

It sounds like a bad master. Took me a few years to figure this out on my car. I had a 1-1/16 bore on manual brakes. My pedal was quick to engage but I felt like I was standing on the pedal to slow the car down. I was always worried i'd end up rear ending someone or rool too far past a red light. I swapped in a 7/8 bore and my pedal travel increased but my effort decreased. The brakes are nice and tight now.

If you are hitting the floor and it's too soft, you possibly have too small of a bore.

[bad86ss]

I went ahead and swapped master cylinders. My old one turned out bad, my NEW ONE was bad too! Master Cyl #3 for the day worked great though. Actually for a 79 Riviera