I have an American Standard Reliant + shower head that drips. Have replaced the cartridge & still dripping. Actually water is slowly coming out of the pressure balancing unit even when the water line is shut off. Do I need a new pressure balancing unit? What are the check stops for (one on each side of pressure balancing unit)? Any help would be appreciated.

Hey Horse... the check stops are actually shutoffs... so you do not need to shut house down to work on valve.

The shower head does not leak... the shower valve does.

Some american standard valves have two cartridges (a pressure balancing cartridge and a ceramic cartridge) that can be replaced. The first one is obvious and you said you replaced that one... the second cartridge is under the first cartridge AND under the three screws you should see holding it in. Remove these three screws and pull out cartridge... clean of all mineral deposits.. and try to reinstall ( I like to purge the water lines by pulsing the water for a second or two on each side (cover faucet with rag) to clear lines of any loose debris/deposits).

If that did not do it then remove again and take to the nearest plumbing supply house to get it replaced. That should do it for you.. good luck.

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I have less experience than MASS with American Standar but I would remind you to grease the valve body before installing a new cartrdige, did you?