The original-equipment master cylinder on your truck has a residual pressure valve built in. If you had to add another one, somebody has dropped the ball while rebuilding the master cylinder at sometime in the past.
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!