You've got a stuck relief valve. Yank it out, and either replace the pump, or do a mirror-polish job on the relief valve with a felt wheel and jeweler's rouge. Check the valve bore for scratches, also.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!