My 1988 Vanden Plas uses the hydraulic mineral oil system for the brake booster. Recently the brakes became soft when pushing on the pedal, and the "Low Brake Pressure" warning appears on the Vehicle Control Monitor (VCM). When I checked it needed mineral oil. After I filled the reservoir with mineral oil and soon discovered a leak from the plug on the side of the reservoir. I repaired the leak by replacing the plug, and then filled the reservoir again with mineral oil.
The brakes are still soft, and the "Low Brake Pressure" warning still appears on the VCM. Although now when I pump the brake pedal twice I get good pressure on the second pump.
I am thinking that air might have gotten in the system from the leak in the reservoir, and that bleeding the brake system will get brake pressure back to normal. What do you think?

 

Yes, bleed the brakes.

Larry