Some showers don't hold a constant temperature very well. You're forever adjusting and readjusting the knobs to keep the water tolerable. This can be very frustrating and takes the pleasure completely out of the showering experience. If you own a shower such as this, don't call a plumber to fix it. That's too expensive. You can save money by doing it yourself in 6 easy steps:
1) Shut off the water line servicing the shower.
2) Expose the plumbing by using a hammer to dislodge any ceramic tile and/or use a reciprocating saw to cut through plywood and sheetrock.
3) Locate the hot and cold water lines and label them as such.
4) Using a 32 oz. fester wrench, disconnect the dorry flange on the hot water line from the positive pressure feedback valve on the cold water line. Using a standard metal integrity sensor, inspect the valve for signs of corrosion, scorching or metal fatigue, and discard the valve if either of these problems exist at a range outside that recommended by OSHA. If the valve is OK, cleanse it with methyl ethyl ketone and reconnect the assembly. If the valve is faulty, but within OSHA integrity ranges, continue to step 5.
5) The faulty valve can be repaired using a two-component bonding resin known as FlowNail. Component A of FlowNail should be at least 45% MDI. Check the Material Safety Data Sheet (MSDS) for compliance. Mix component A and component B in a plastic cup using only a wooden tongue depressor. But first, put on nitrile gloves and a positive air purifying respirator (PAPR). Now apply liberally to the valve making sure not to clog the threads. Let dry 72 hours before reconnecting the plumbing assembly. In lieu of repairing the valve, a new one may be purchased from NASA once the appropriate permit application has been filed.
6) Enjoy your shower.