PA-42 pilot reported a very long takeoff roll related to parking brake remaining partially engaged.

Narrative

Takeoff from 2300 ft. strip; twin engine aircraft; have flown many takeoffs from this runway. Released parking brake; aircraft begin rolling; everything appeared normal. However; as speed increased; while aircraft was still accelerating; acceleration was felt to be not as strong as normal. Checked engines and props; all appeared normal. By this time aircraft was at 80 kt.; should have been around 90 kt.; too far down the runway to stop; knew I could not stop and the only option was to continue. Used entire runway; rotated at the end of runway at about 85 kt.; rolled off the end the runway. Runway had a slope off the end; was airborne for short distance; touched down lightly in the dirt off the end of the runway; and then became airborne in controlled flight. Once airborne; aircraft performed as usual.Looking for a reason for the problem; looked at parking brake handle; which is a small T-handle low on the instrument panel. It appeared to be in the forward; off position; where it should be; but when I pushed hard on it; it moved forward about 1/16 of an inch. Then I recycled the parking brake; pushed on it in the normal manner; it stopped in the normal place. I then again tried moving it further forward and could not move it any further regardless of how hard I pushed. Repeated 3 more times; same result. Inspected aircraft after landing; no damage; all normal. Cycled parking brake numerous times on ground; could not get the problem to repeat. Before this flight; the left brake was weak and Mechanic serviced the hydraulics for the brakes; per the maintenance instruction. I do not know if this had anything to do with the problem or not.Cause of sluggish acceleration was a partially dragging parking brake; and the drag was not evident until it was too late to stop. This aircraft; and many aircraft; have very poorly designed parking brakes as they are a simple push-pull with little travel and much 'sponginess' and there is no 'feel' to them to guarantee that they are fully off. To avoid any incidents; they should be designed with positive on-off detents that guarantee that the parking brake is either full-on; or full-off. There is never a need for a 'partial' parking brake; and this aircraft would be very difficult to move if the parking brake was full on. When I released the parking brake and started rolling; I fully expected the parking brake to be fully released; period. This is a serious design flaw and parking brakes should be designed so that they can only be in the full-on or full-off position; with positive detents for both.