How to pressure-bleed the brakes on a B-van?

Remember how on the early B-van, the brake master cylinder was mounted sideways, and so close up under the body that you could barely get the top off of it?

The brakes on my van have gotten pretty bad. I don't seem to be getting any pressure to the rear brakes, and the pedal is low. I would like to be able to hand pressurize the M/C with my Mity-Vac in order to diagnose the problem, rather than having to ask someone to pump the pedal, but I can't figure out how to do it, due to lack of clearance between the top of the M/C and the body.

I was hoping that some of you kind people would advise me how to pressurize the M/C on my vehicle. Thank you very much.