The fluid level in my MC front reservoir drops over time. I tried the 30 minute on the brakes with cardboard under the car w/o any visible leaks. Any other diagnostics I should try to narrow this down?

Places to take a closer look are:

1: Brake switch under the MC. Depends on which model S30 you have several types of Brake Switch. The O-rings can leak. Sometimes leak through the switch o-ring. Its also behind a heat shield which makes it hard to get a good view.

2: proportioning valve can leak from the o-ring. Its located on the firewall on the 280Z and behind the diff on the 240Z

3: wheel cylinders. Check the inside of the wheels and the backing plate for fluid and while your there, check the hoses for cracks and age.

4: check the brake lines where they go through the rubber insulators along the chasis. The rubber is very old and hard. It can rub through the brake line.

Goodluck finding your leak.

Chas

Sure. Put some fluorescent dye in (used to diagnose AC leaks). Drive it for two weeks. Pull into a darkened garage, at night, and use a black light to find your leak.

If you don't find it, it's probably leaking into the power brake booster.

Booster is my first concern. I would think an external leak would show up on the floor.

C

How much fluid over what period of time and milage? The fluid level for the front will drop as the pads wear.

Thanks.

Is there a way to confirm short of dissembly?

What was the answer to Beerman's ???'s how much ? how often? If your rear drums are dry and your pads are not very thin. then I would suspect the booster. The master cylinder can be pulled away from the booster. If the end of the master cylinder is corroded it has probably been leaking. The only way other way to check is to open up the booster or take it off the car and see if any brake fluid will drain out the vacuum port.

C

If its leaking into the booster, then it will be coming from the MC rear reservoir. Hls30guru stated the "MC front" which I presume is the front of the MC for the rear brakes. In that case I would be looking at the rear wheel cylinders and go from there.

The front brakes will use fluid when the pads wear, but the back brakes wont because they return to original position when released. Brake shoes wear is mechancally corrected by an adjusting screw when you use the hand brake.

Can you explain what was involed when you did your 30 minute test? If you are just pushing down the brake pedal without the engine to provide vacuum assist, you probably wont get it to leak.

Chas

You need to be clear about what you mean by front. Do you mean the front brakes or the fluid reservoir that is toward the front of the car? Most cars have the front brakes connected to the reservoir toward the rear of the car. The front brakes generally have a larger reservoir on designs that the reservoir is not common to both the front and rear brakes. I believe that Datsun did connect the front brakes to the forward reservoir in the first year or two of the Z so it is possible that in your case the front brakes are connected to the forward reservoir. What year is your car?

To answer you question, you must remove the master to see if it is leaking into the booster unless you can see signs of a fluid leak on the oustide of the booster near the joint with the master. Look for bubbles in the paint on the booster just below the master. Make sure it is not from fluid you may have spilled while working on the car.