jam,
just relieving the pressure off the master cylinder will not fix the problem bbforks was pointing out. All brake systems require a certain amount of 'freeplay' in the pedal linkage/mastercylinder.
the way to check this is to lightly depress the pedal and see how far it moves before you feel the rod start pushing the piston in the mastercylinder. There is a very distinct difference, so you can tell when the adjusting rod hits the piston. That travel should be about 1/4 inch and no more than 1/2 inch movement "BEFORE" the adjusting rod touches the master cylinder piston. This is called "freeplay". If there is no freeplay the system will just keep pumping itself back up again because the piston inside the master cylinder is not returning completely as bbforks explained earlier.

I would definatly say this is causing the rockhard pedal and brake lockup.

Now as for this 'checkvalve' lift1mech is talking about? i have seen those in older lifts, they were commonly used to help the operation of the system but in todays master cylinders they did away with this valve. So unless this unit has the origional master cylinder or if it has been rebuilt and the kit had this check valve in it i wouldnt count on this being the issue. Worst case scenario if this valve went bad is just a mushy pedal from air or something. I suppose if it became stuck it may lockup the system but i have yet to see this happen in any of the ones i have dealt with, generally they deteriorate and just become ineffective.