Despite it having big AP calipers & discs, I've not been happy with the brakes on the Libra since I got it, so latest project has been trying to sort it out. Doing some research I came across a few posts on here re. similar issues, so my findings may be of interest.
My problem was an inconsistent brake pedal with an occasional hard/dead feel - not very often but it caused a few sweaty-palm moments,
so had to be fixed.Tried all the usual stuff (bleeding, etc.) to no avail, so I was beginning to blame the "Yellow Stuff" pads for being too hard for road use - not a very convincing theory as the brakes were acceptable most of the time but I was running out of ideas.
Then I recalled a similar pedal feel from decades ago when I had a servo develop a small leak, so I focussed on the Libra servo.
I thought perhaps there was a leak in the long vacuum pipe down the tunnel but when I hooked up a vacuum pump and gauge to the engine end of the pipe, it held vacuum. So I crossed that off the list of possible causes and started looking for a replacement servo in case that was failing under braking.
Then I gave it a bit more thought and decided to remove and check the non-return valve in the vacuum pipe - Bingo! It was no longer doing its non-return duties, so I swapped it for a spare and the brakes are now terrific.
I presume that when the engine was generating high vacuum the failure of the valve was of no consequence but when there was little vacuum available the servo-effect was reduced or absent all together - just depended what the engine was doing when I was braking.
So it's worth checking the little black and grey valve in the vacuum pipe if you have similar issues.