Have you fully checked your brake system ?
(Calipers, hard lines, flexible lines)Have you flushed the brake fluid recently ?
If the problem is really the master cylinder, then it's time to consider one of the replacement units, as the stock brake master cylinder is not currently rebuildable.
If your booster is useable, you can mount one of several masters to an adapter, or you can replace the booster and master as a unit.
I did this, and wrote an article on how, appearing in the next Pantera International.
Chuck
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.