I have a '73 240z. The brake pedal goes all the way to the floor before the brakes will engage. The emergency brake light goes on when I press the brake pedal all the way down. The brake fluid reservoir has fluid in it. Is there a way to check the master cylinder to see if it's bad before I buy a costly replacement? It worked when I bought the car and I haven't driven it probably more than 500 miles in 5 years.

Search "reaction disk". By your description, that sounds like the problem.

Usually if a brake master is going bad, the test is to stomp on the pedal and it will hold pressure, but if you put a light pressure on the pedal it will slowly sink to the floor.

Unless the m/cylinder has been disassembled from the booster it's unlikely it will be the reaction disc. First check for leaks at the brake cylinders and calipers. If there are no leaks then pump up the pedal by a few strokes and if it pumps up then goes to the floor the chances are the m/cylinder is at fault.

Also no need to replace it with a new one. Have the old one stainless steel sleeved.

What happens if you pump the pedal? Does it firm up and hold or does it always go right to the floor? If it pumps and holds, you have air in the lines and they need bled out at the wheel cylinders/calipers. If it pumps up solid for time and fades away as you hold pressure, check on the inside of the wheels and on the floor/driveway for brake fluid. A wheel cylinder or caliper is leaking. You will have a loss of fluid in the resevoir as well. If no fluid loss, the piston in the master cylinder is bad. Could be as simple as replacing the piston after a light honing of the cylinder. Likely a replacement master cylinder is faster and as cheap. It will need bled again after bench bleeding it, starting with the wheel furthest from the cylinder on each circut (front discs and rear drums).

Let us know.

Leonard