We're having trouble getting all the air out of the brake system on a 66 manual drum brake car. We replaced the master cyl, vent hose on the rear axle and the brake pads. Wheel cyl are not leaking and no visible leaks on any lines or fittings. Is it possible that we are sucking air through the rear brake vent hose on the rear axle. Is that a one way valve in there? I bleed each wheel starting from right rear, left rear, right front left, left front until no more air bubbles are visible. Pedal will firm up temporarily but will quickly give out and go to the floor. Bleed again and come up with the same issue. We've attempted this 6 times to no avail. Any help would be appreciated.