I'm using a dual master cylinder and proportioning valve from a 1985 S-10 Blazer. Big reservoir is rearward and feeds the front disk brakes. I don't know if this is expected but I also don't understand it...when I push on the brake pedal with the reservoir cover off, the rear reservoir squirts upward big time, enough to get all over the place. I've looked at some diagrams of typical master cylinder cross sections and all I can think of is that the primary piston seal is leaking bad, allowing fluid destined for the front calipers to leak backward and squirt up the reservoir. This happened on the original cylinder I used and also on a new "made in china" master cylinder from my local O'Reilly's. The primary piston is supposed to push the secondary piston (to rear wheels) and I don't think this is happening based on the fact that I'm having a hard time getting brake fluid to the rear axle. And for some reason, I can't get fluid to the rear axle even with my Mity Vac. I'm confused.
