I recently changed my master cylinder on my 81’ pickup. After bleeding the whole system the brake pedal is still stiff as a rock. Checked the brake booster check valve and lines, and brake lines. I don’t know what else this can be. I took it in a while back and the tech told me it could be a brake booster, but now that’s the hard one-they don’t make them for my 81 anymore!!! NEED HELP PLEASEEEEEEEEEEE