Maybe check that the brake cables are properly seated at both ends. It is possible to end up with them not properly in place. Tug the cables at both ends by hand and try applying the lever and letting it go again to see if the cables will slip back in properly. Also, squeeze the blocks against the rear rim by hand (literally with your fingers on the blocks) and wiggle the cables at the caliper end, and at the lever end. I'd suggest that the trouble is with cable seating since it was ok before you folded it up. If you don't have enough slack to make any changes, try adjusting the handlebar end of the cable by turning brake adjuster inwards where the cable comes out of the brake lever. this might give you enough slack to get things back where they should be.
Once you've fixed it, I'd suggest you carefully fold her up again, paying special attention to what happens to that cable so you can see how it all went wrong. You should be able to see the fault develop as you fold the bike slowly while watching how the process interacts with the cable that went wrong.
Good luck and send feedback when you solve the problem.
Last edited by EvilV; 09-01-07 at 09:21 AM.