Brake Fluid Leak
 

I have a very slow, but mysterious brake fluid leak in my 93 FD. Ive inspected the calipers, brake lines, brake master, clutch hydraulics, and pretty much everything else. Everything that should be leaking is bone dry. The only clue i have is that it leaked out and left spots on the floor underneath the car on the passenger side of the car, about half-way between centerline and the caliper. Note that the caliper and the brake lines, abs, booster- everything is dry. I'm kind of pulling my hair out and spring is fast approaching. Anyone have any advice? is the booster failing internally or something?

brake boosters don't use brake fluid. you can unbolt the master and get a peek behind it and see if the fluid is pushing out the back INTO the booster. it would be obvious if it was but you can check anyway.

what you will need to do is have someone pump the pedal until it is rock solid and then they will STAND on the pedal. while that's happening, you will inspect and find the leak

Easy way to DIY pressure test is to grab the handle from your jack or a length of 2x4 and jam it between your brake pedal and seat so that the brake pedal is pressed and the system is under pressure. From there wait for the leak / seapage to start and observe carefully. Sometime feeling the junction between brake lines for dampness is more successful in finding leaks than visually inspecting.

Good luck.

Fwiw, I've seen leaks from the bleeder screws and copper crush washers so pay particular attention there - and to the junction between the brake line that goes from the caliper to the hard line

Also, don't forget to check the clutch slave cylinder as well. Same fluid.

Unfortunately, i let it sit a month or so and ALL the fluid has leaked out. So im going to try and re-fill the system this weekend. Its clearly none of the calipers, connections, soft/hard lines, the master cylinder or the booster it seems. I mean, i had full brake fluid last summer, it had to go somewhere? Where else could it be leaking out of or into?

You're missing something because you named all the places lol

If your master cylinder went dry and sat, theres a good chance it will need to be rebuilt. After going dry, they have the tendency to just not work anymore. It will manifest as an air free bleed but a very mushy pedal. Something to consider...

All the brake fluid leaked out as in the reservoir is dry , or it was all the way down to the nipple that comes off the side? If the level stopped dropping at the nipple the problem exists somewhere with your clutch hydraulics. IF the reservoir is dry check for leaks under the ABS pump, the paint will be visibly peeling under it even if the fluid has dried up.

Bone dry underneath the ABS pump, bone dry under the booster, the master cylinder, all connectors, all calipers- dry. It makes no sense, like the fluid just magically disappeared.

As others mentioned, how about the clutch hydraulics?

The most mysterious brake fluid leaks I have had, have been the rear calipers. I have had more than one leak through the e-brake mechanism. They would always look and act just fine but lose fluid very slowly. One time the only trace of fluid was on the rim between the inner lip and tire (never made it to the floor). Another time I only tracked it down because it stripped all the paint off the inside of the barrel, haha.

I feel like a whole reservoir should have left some clearer signs, but ya never know. Did you check the inside of the car? Clutch master can leak down the inside of the firewall.

I will double-check the rear calipers tomorrow. What was leaking on em? like the line leading to em or....?

As far as I could tell, mine were leaking from the e-brake lever/shaft that goes into the caliper body. In both cases the calipers were rebuilt units, so probably just poorly done.

You don't say which caliper (or LH/RHD).......with the booster mention, I might assume that's happening at the front of the car?

The reservoir emptying in a month, it wouldn't be a particularly slow leak in my book either. With the clutch slave the lowest point, somewhat hidden and a common corrosion failure, I'd pull that (or do a lazy check of the bellhousing inspection cover at least) if that's in the general vicinity of the oil spots..

Out of all the places you've checked, there is one more that is really common and easily overlooked.

The Clutch Master in the Drivers Footwell.

Check around the top of your clutch pedal and see if there's any yellowing or smear marks running down the firewall.

This would only affect the rear fluid chamber (The brake Master has a wall in the centre of it to separate the front brake fluid from the rear brakes + clutch master)

EDIT: My apologies, you mentioned finding spots on the passenger side. Try using an LED torch to look for a wet trail under the body or for some spray. The information about the wall in the Master may help you identify if it's the front or rear. If it's both, then you may have more than one leak in another place. You could also refill the Master with fresh fluid, place a piece of wood or jack handle on the brake pedal after a few pumps.and then look around for the leaking line while it's under pressure

Gracer7-RX7's technique is a good one. It'll turn a slow leak into a fast leak which is much easier to find. Here's some pictures of how I used this technique to track down my leak (which I'm still %@#&! fixing, yeah spring is here and I'm still on jackstands):
https://www.rx7club.com/3rd-generati.../#post12591434