So, my "new" 73 has a new master cylinder and the other day I found it a bit hard to stop. Peddle is rock hard. Pulled the vacuum line imagine bay side.n it let out a Lil wet vacuum noise. Cabin side I can hear a squishing release sound. The kicker is, i'm not losing fluid into master. In fact it's,very full. Any ideas?

It sounds like the Master-Vac failed. A quick test to see if it is working:

With the engine off pump the brakes a few times. This will deplete any vacuum remaining in the Master-Vac.

Press and hold the brake pedal down with moderate force.

Start the engine (while still holding the brake pedal down).

The brake pedal should drop a bit as the engine starts and the manifold vacuum begins assisting.

The factory service manual has additional diagnostic procedures. If you do not have a copy you can get it here: http://www.xenons30.com/reference.html

Thanks so much. I'm on it.