I had one on the roadrunner prior to taking it all apart, 4 wheel drum. I don't recall where it it was plumbed in but I also had pumped the brakes a few times before hitting the button. It held when doing this. I am sure you checked you fittings but do you see any seepage of brake fluid anywhere? Try pumping a few times as A39 mentioned. Also, does the peddle move to the floor after holding it down? Are the wheels locked and it is just sliding on the tires?