Brake bleeding
I have a one-man Gunson brake bleeder that connects onto the tyre and pressurises the brake fluid to bleed the brakes. It does an OK job, but have you any idea why, on two occasions, when I released the pressure pipe from the tyre, the bleeder sucked the remaining fluid from the reservoir bottle up the pipe and released it everywhere? I had to get a basin of soapy water to wash the fluid off the tyre and under the bonnet to prevent lasting damage. I had the same Gunson bleeder back in the 1990s and it never gave trouble, apart from getting worn out, which is why I bought this replacement.
The pipe which connects to the spare tyre should only go into the cap of the bleeder bottle and the pipe to the reservoir should go to the base of the bottle. The recommended pressure to use for bleeding is 20psi. Unless the bleeder bottle was particularly full or the spare wheel was at too high a pressure, I can see no reason why the fluid would push back into the pressure hose and out of the valve attachment.
As brake fluid cannot compress, the only compressed pressure should be the air in the system. The only scenario I can think of where this might happen would be if there was still air in the brake system which was then compressed. When the pressure was released that air would force the brake fluid back out and into the bottle. But even this should not push the fluid up out of the top of the cap and through the tube which connects to the spare wheel.
It is worth noting that many spacesaver spare wheels do run at a far higher pressure and, if using the Gunson bleeder with one of these, the pressure should be dropped significantly.