Originally Posted by Redwood
Fix the faucet to stop the leak. Cartridges, or, washers & seats whatever it takes. Don't strip out the seats using an improper tool!
Flatheads, hammers, & Channel Locks are not the proper tools and may turn a small inexpensive job into a large expensive job!
Please read How To Replace Faucet Washers
before doing anything. The article shows the proper tools and discusses the proper methods to replace washers and seat in a faucet.
Placing the valve on the shower arm and having both hot and cold valves open allows the hot and cold water to crossover and affects the temperature on the other fixtures.
If the seat is cracked then it needs to come out if you go to the local shop or HD you will have to buy a pack of the seat fitters all different sizes this can add up $$$
I have about 5-10 different size seat pullers half the time they don't fit that's when my wide flathead comes in 100% I get them out.
Channel locks are a plumbers best friend.
Oh yeah if the stem is resest into the wall then there you will have to buy a pack of them the metal sleeve covers.