If it is leaking when the pump is on and not at the toilet valve itself that would eliminate the vacuum brake. My vacuum brake only leaked when flushing. If you do not see water dripping from the toilet valve itself then I would think it is one of the crimp connections on the 1/2" pex line. I had to replace a bunch of the crimp connections on mine. The originals were plastic and tended to crack. I used push to connect fittings from Sea Tech or Watts for repairs. I had to order them online. You can also use sharkbite brass fittings from Lowes or Home depot. be prepared to follow the Pex lines from the toilet/bathroom sink/water heater/shower/etc to find the leak.