Leaky adjustable proportioning valve

Some would have to leak out before the reservoir gets enough vacuum to hold it back. The fuller it is, the less will leak. I doubt any liquid sealant will work unless the port is perfectly dry and clean. Teflon tape would still work. It is used to seal 10,000 psi hydraulic fittings. Your brake system is <1000 psi unless you have a linebacker's legs.