Take the oil dipstick out and see if it's blowing pressure out through there as well. If this is the case, than you may be getting coolant in the oil (bad stuff for the motor).
I started the truck on sunday,letting it warm up for about ten minutes, it never got warm and puked coolant from the reservoir.never starting it I went to open the res after sittin for two days and the pressure blew the @#$% cap out of my hand and sent an explosion of ice cold coolant everywhere, I had to change my @#$% clothes! There must have been 30 psi in the system