I would have changed the brake master cylinder first,basically if the calipers show no signs of leaking then they are ok.As for the master cylinder weak piston seals can cause failure without any signs of leaking.

If you are sure it's bled right, and you are not losing fluid, it can only be the master cylinder. All else being correct I assume. Sometimes, it's real difficult to get all the air out. But if you have a pedal and are losing the pedal occasionally, I would look back to the master cylinder for sure. The booster would only give you a hard pedal if it was bad. You might just for grins and chuckles look at the point where the master cylinder goes on to the booster. At least just make sure.


Good Luck

Was there signs of fluid on / in the old booster? Why did you replace the booster? I ask caust the new booster may have an internal vacuum leak and be "suckin" the fluid out of the master into the booster housing.....

For what its worth...

Daryl

I replaced the calipers because of rusted and broken bleeder screws on all of them. I replaced the brake booster because it had a hole in the bottom.

You either got air in the lines a bad master clyinder or one of your break lines is leaking.e