I just bought a foreclosure and we had to replace the rusted non-working water heater. I think because of it the cartridge in the bathroom got clogged with rust and sediment and no hot water was coming through. I removed the valve, blew some water through the line and replaced a moen shower cartridge valve. The hot water now comes through the spout, but now there seems to be some back pressure pushing the water up to the shower head.
The only thing I could think that may be wrong is that the replacement valve did not have a back o-ring only a front one. I tried to use the old o-ring on the new one, but the groove was too narrow. What else could it be?

-Trent

I assume this is a Tub/Shower faucet.

Did you remove the whole valve assembly or just the cartridge?

Blowing water through the lines would not help, you need to flush the faucet, to do that you would need to shut water to the valve, remove the cartridge turn on the water for a few seconds to flush out all the debris out of the valve, clean the cartridge and install see if the problem is fixed.

Can you tell us the model number of your faucet?

John