Any tips for removing calcium buildup in the valve (manifold) of an old two handle (Hot & Cold) tub/shower?? Water flows readily through each stem seat, but is slowed to half while going thru the valve (manifold) before entering the bath spigot. Have had limited success twisting a wire into the area. If I must tear out the wall and replace the valve, I must. But, I'd much rather not (tile matching problems etc.) Thank you for a prompt response.