Brake pushrod length.... and other brake woes

The geyser is proof of either
1) the rear shoes are way out of adjustment, and the return-springs rapidly retract the shoes which drives the pistons back, which drives the fluid back.
2) big time air in the system, which acts like an air-spring.
Those are the only two I have ever experienced .

You can lock the rear wheel cylinders by C-clamping them.