G Klasse Brake Issues

Spongy brake pedal and squeaky brakes on a G Wagen? That's not a good sign. Undervoltage causing brake issues is far-fetched. More likely you've got air in your brake lines or your master cylinder is going bad. Does the pedal sink to the floor if you hold it? Have you checked the brake fluid level? Low fluid can cause that spongy feel and affect the brake shoes too. And check those brake pads again, sometimes visual inspection isn't enough.

Thanks for the input. I'll check the fluid level. When you say air in the brake lines, how does that happen exactly? Is it something that just occurs naturally, or is it more likely a sign of a leak somewhere in the system?

Air in the lines isn't usually a 'natural' thing. It gets in there when you open up the hydraulic brakes system, like during a brake job if you don't bleed them properly afterwards. Or, yeah, a leak somewhere is a big possibility. If the fluid's low, you've GOT a leak. Better get it checked by a pro before you end up with zero brakes. Brakes are not something to mess with. The squeaking could also be unrelated, just cheap brake pads or some rust on the discs, but get the main issue sorted ASAP.

Okay, I appreciate the advice. I took it to a workshop, and it turned out the master cylinder was indeed faulty and needed replacement. I also got new brake pads while they were at it. Total cost was around 240€. Thanks again for pointing me in the right direction.