Master Cylinder

[snottyone]

I have a 94 surf auto and having some issues with the brakes. If I have to come to a panic stop and slam on the brakes, the pedal is rock hard and barely moves and only the front brakes are working. If I use them softly and normaly, it seems all brakes are working. Not sure if it is the LSPV or the master cylinder. Since I am in Canada, masters are hard to come by, but I can get a rebuild kit for it. Or should I try the LSPV first? Thanks for any input.
Scott.