I've only seen it happen once, but a defective brake hose with a frayed piece inside the hose caused similar problems. Applying brakes would force the frayed material through the fitting, but when released it would act like a check valve and prevent fluid from returning to the resevoir. Also if you have a hydrovac, if the spring is installed on the wrong side of the small diaphram (don't ask) or crud inside is interfering with it you will get the same effect.
