The water pressure in one shower has dropped to about 10% of what it was originally. I replaced the shower head but it didn't make any difference. The mixer valve is about 24 years old. Water delivery is controlled by a push/pull of a large hand knob. Temperature is controlled by rotating the same hand knob clockwise or counter clockwise. While the knob rotates freely, it has become difficult to move in and out, so I assume this is the problem. Is replacing the mixer valve a job for a professional?