My brake pedal felt lower and spoongy than normal when driving. Checked master cylinder reservoir and found it to be down in brake fluid about half way. I refilled it and checked all 4 wheel cylinders and brake lines under the car and all were dry. Test drove car again and pedal was still lower and spoongy. Is it possible that master cylinder is leaking back into the vac. booster? Or is my booster going bad? Or could both be happening. Vac. boosters are hard to come by for my 79zx. Any thoughts?

4 hours ago, mayolives said:

Is it possible that master cylinder is leaking back into the vac. booster?

This is possible. Unbolt the MC from the booster and pull it forward a bit. Fluid will leak out if the rear seal of the MC is leaking.

If you do find a leak you might be able to save the booster if you clean out the brake fluid from inside. Not sure what the best way to do that would be. Maybe water, since brake fluid and water are compatible.