Why does Oil light come on when you use the brakes on a 1981 Toyota?

Oil light indicates low oil pressure.

When the engine revolution decreases, such as during braking or idle, oil pressure drops. If the pressure is too low, oil light will come on.

Possible causes:

Most likely cause is a leaky oil-pressure switch. If you see that oil is leaking from the oil-pressure switch, you need to replace the oil-pressure switch. Tighten the new switch to the correct tightening torque specified in the repair manual but do not overtighten. A leak at the oil-pressure switch causes the oil-pressure switch to give incorrect low-oil pressure warning (oil light) because the leak causes the pressure to drop around the oil switch. If this is the case, it would be easy to fix the problem.

If this is not the case, there may be other oil leaks in the engine or a faulty oil pump or other engine problems.

Also, obviously, make sure that your engine is not nearly stalling during braking, which would also cause the oil light to come on due to very low engine speed.