I'm not sure about the mustang, but generally there is a little lever that slides into the mechanism as you pull the handle up. That is what causes the buzz when you apply the brake. When you push the release on the handle, the lever tension is released and the handle goes down. Check that the lever engages fully. It may be worn or need a little lube. The release button may be sticking in the "release" mode just enough to not allow the lever to hold, or something may require a little lube to engage it, or the spring may be tired. It should be a fairly simple mechanism. Look for extra slack and find the source.