The brake light comes on when the system pressure is low. If it takes 10
minutes of highway driving before the light goes out, the bomb is not
re-charging fast enough. This can be caused by restrictions in the lines, a
weak pump, or a really leaky servo. Even if the bomb is bad, the system
should pressurize rapidly. Ditto the hard pedal after repeated brake
applications - either the bomb is bad and not storing enough charge, or the
system isn't re-pressurizing the bomb fast enough.

I'd suggest the following:

1. Test the bomb: Run the engine until the brake light goes out. Shut the
engine off. Pump the brakes until the pedal gets hard. Count the number of
pumps - you should get at least 10. A new bomb gives 30+.

2. Test the servo: Run the engine until the brake light goes out. Shut off
the engine. Remove the servo return hose at the hydraulic reservoir.
Pentosin should not be coming out of this hose. More than a few drops
indicates a leaking servo which is discharging the bomb back into the
reservoir.

3. Test the bomb check valve: Pressurize the system by running the engine
until the brake light goes out. Check the Pentosin level in the hydraulic
reservoir. Let the car sit overnight and check the Pentosin level again. If
it is higher and the servo is OK, the bomb check valve is leaking, allowing
Pentosin back into the reservoir and discharging the bomb. This is a common
problem in the S-cars.

4. Discharge the pressure in the system by pumping the brakes at least 40
times with the engine off. Remove the banjo bolts in the hydraulic system
and examine closely. Some of these bolts have a fine internal screen in the
oil passages. The interior of the hydraulic hoses can break down with age,
and the rubber particles clog these screens, restricting oil flow, which in
turn prevents the bomb from recharging rapidly. This is a good opportunity
to flush the Pentosin if it looks dirty. Make sure you remove and clean the
screen in the reservoir.

If the bomb & servo are OK and the lines are clear, your pump may be weak.