Please review the "Important Information" notes of the Installation Manual for the K-304, K-8304, K-305 or K-11748 valves. The note indicates that only 1/2" copper pipe (type L or M) can be used between the valve and the bath spout. Kohler recommends this for the connection to the shower head as well.
Flow problems: Using materials other than recommended may cause back pressure and cause water to flow from the shower head and tub spout at the same time.
Delayed flow: When the plumber installs pex tubing between the valve and the shower head or tub spout, you may experience delayed water flow from the shower head/tub spout. The problem is caused due to thermal expansion of the pex tubing during bathing. Once the user shuts off the valve and the pex begins to cool, it contracts causing the column of water to seek the path of least resistance, which is the spout or shower head.
Additional Resource
K-304 Installation Instructions
