If it's play in the pedal, then there are number of things to look at way before you replace the prop-valve. First off, you're talking about excessive pedal travel right? Not a floppy pedal... A floppy pedal probably says that something is wrong with your pedal cluster.
1) Bleed the brakes. This should be step one whenever diagnosing brake problems.
2) Inspect pads and rotors. If there is excessive wear, this will increase pedal travel.
3) Check rear brake adjusters. If they are way out of adjustment, you'll notice extra pedal travel.
4) Inspect the flex lines at the calipers. If they're really old rubber lines and are obvioulsy cracking and/or showing sinnes of wear, then they may be in need of replacement. Even new rubber lines will make a big difference over worn out cracking ones. The SS braided lines are nice, but you should inspect them regularly, as they won't give much warning in the unlikley even of failure. Rubber ones fail more gracefully most of the time...
5) Look at the master. Especially the mount. Check to see if there is a lot of flex in the mounting. Have someone press the brake pedal while you look at the master from the under the car. It shouldn't move. If it does, then maybe the bulkhead has weakened. This problem can be fixed quickly, cheaply, and easilly (if only they all could right?). There's a kit that one of our members has for sale.
Here.
6) Make sure the master isn't leaking.
7) Replace, or upgrade the master to a 19mm. Upgrading to the 19mm master will result in a pedal with less travel, but will require more effort for the same braking force. Basic physics/hydraulics in effect there.
8) Replace the prop-valve...
After that, you're getting into "big brakes" and stuff like that...
How's that?
-Josh2