I've had a chronic small leak (apparent by a pressure loss) in the 7/8" pex tubing that I used for a radiant heating system. The 700+ ft. of tubing is inaccessible from the top under a hardwood floor and 3/4" plywood. The tubing is inaccessible from below as it is installed over 3/4" plywood which is over insulated 2x12s which is above an older concrete slab that was formerly a patio. The pex was purchased from The Radiant Floor Company in Vermont and was pressure tested before covering, though after one nick in the pex was repaired using barbed couplings secured by hose clamps. Radiant has not been helpful in solving this situation. I've tried "Silver King" boiler additive with no success (the manufacturer said it would work with pex). Is anyone aware of a product that would have the ability to seal this leak and hold it within the operating temp (up to 200 deg) and pressure range (12 to 15 or 16 psi) of this system.