First, my attempt to keep this as simple as possible:
I am getting constant flow out of a pressure relief valve: not high pressure blasting out but a consistent flow of about 16 oz. per minute (…unless any tap is opened).

Other than water pressure exceeding 150 psi or a faulty valve, what could account for this?

(I hope I'm wrong but my hunch is that this will not be easily answered without all the details so I'll just get started on writing up the specifics & check back later.)

Thanks