'89 GMC medium truck

Anybody familiar with the hydraulic braking system that ties into the power steering pump on this truck?

yeah why?

FIL has a box truck. The other day, his partner said that he hit the brakes and had next to no braking power. He opened the hood and fluid was everywhere. I took a look at it, and when you step on the brakes, the fluid is shooting out of the pressure relief hole (?) on the lid of the power steering pump reservoir. Does this mean that there's a blockage somewhere in the brake system?

does it also affect the way it is steering? or just brakes? was it over filled? how old is the fluid and has any of the lines/pump/box/booster been replaced?

I'm not sure if steering was affected, they didn't mention that. I checked the levels on the steering reservoir as well as the master cylinder reservoir, and both looked ok. I'm sure the fluid is very old and no parts have been replaced recently.

try flushing the fluids. then start with the lines.