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.

Thank you Zed Head. I'll give it a shot.

That’s good advice but you didn’t warn him about the disc dropping to the bottom of the booster.

@mayolives do yourself a huge favor and research the disc I’m talking about located on the the end of the pushrod inside the booster. There are rebuilders for that booster.

Edited July 18, 2025Jul 18 by Yarb

Called a "reaction disc"

Yarb and Patcon, Both mentioned the "reaction disk" and that I needed to be carefull how I delt with it. It's been many years since I replaced a brake booster on a Z car and I don't remember having to deall with the "reaction disk". I searched my FSM and can't find any mention of a reaction disk. I haven't found it listed on any of the drawings either. Please tell me more about it. Also, where can I find a rebuilder for my booster? I'm in SC.

Myself and others here on the forum have had great success with Power Boosters Exchange. They are located in The North West. Rebuilt my original and they also can powder coat it for you.

Another plug for PB Exchange, They moved up to Lynnwood WA, I had them rebuild my early booster.

Reaction disc pic's

@mayolives Curious why you are calling your car a 79 280Z? Unless you went with the 79 ZX larger booster.

Edited September 3, 2025Sep 3 by Yarb

Typo on the 79z. It's a 79 zx coupe.

That’s good to know but there are quite a few different components compared to a S30.