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

 

Alright, just a quick update, bled the brakes, got some air out but not much. Took it for a spin around the neighborhood, and made it a block before all braking other than Ebrake failed. The fluid in the reservior also disappeared. Replaced the brake master, and brake booster. Now i have basically zero brakes. I think its a vacuum leak, and if so I hope this damn car burns to the ground. Im so sick of weird nonsense, or things not being the obvious things. What else could it be? Ive replaced the booster, master cylinder, and clutch hydraulics at this point. I have no idea how to proceed other than replacing the vac line from the UIM to the booster.

 

If all the fluid in the reservoir is gone, then you have a leak, plain and simple. It doesn't just evaporate. Definitely check for leaks, it has to be a pretty big one if it was all gone in that short of a trip. The back side of the master can leak inside the booster as well and be hard find unless you pull the master off.

 

sorry to hear this is a continuing issue. Suggest you redo the brake pressure test previously mentioned.

 

Well I replaced the booster and master BECAUSE i kept losing fluid. Now I dont lose fluid, but I dont have brakes either. can a bad check valve leading to the booster do this? I have some brakes, but it takes a few seconds to build pressure and it goes right back to the floor after a second or so.

 

You previously stated you did lose fluid...

I don't think a bad check valve in the line between the UIM and brake booster would suck fluid out of the reservoir. It is attached to the vacuum booster. If it were sucking in fluid somehow, you would hear the engine try to stall as it is ingested. I think you can detach that hose and try blowing throw it in each direction to determine if the valve is working. The FSM might have a procedure describing how to do it using a special tool but the process will probably involve blowing air into each side or applying vacuum to each side of the hose.

When you are feeling less frustrated, do another round of troubleshooting. Jack the car up. Put pressure to the brake system like I previously described. Check the back side of your wheels for any residual brake fluid. Check the caliper bleeders for the same. Take a good look at the inside of the calipers to try and find any leaking brake fluid from a bad caliper piston seal. Check your brake soft lines. Check every junction on the hard lines from the master to to the caliper looking for fluid. Tedious but not hard to do.

Good luck.

 

Ok, I just pulled the hose with the check valve. Can only blow one way thru it so that's working. I initially thought it was leaking because the fluid in the reservior would vanish, but there were never any puddles other than motor oil from a slow oil pan leak. So, I was like...well it has to be going SOMEWHERE. After my first bleed I hopped in it to take it around the block. Lost brakes within a block, the pedal just went to the floor and I had to use the Ebrake to stop. Once I rolled to a stop I checked the fluid level in the reservoir- it was empty so the fresh fluid I had just put in minutes before had vanished. No puddles no nothing. So....I replaced the master cylinder and booster. Still no brakes, same exact feel and issue. I have brakes for a pump or two randomly, the rest of the time its like I have brakes for like a half second then they go to the floor, i wait a few seconds and try again- I have brakes for like half a second and they go to the floor.
So in summary I have:

• Bled brakes multiple times
• Replaced the Master Cylinder
• Replaced Clutch hydraulics
• Replaced Booster
• Checked vacuum line and check valve- both seem fine.

Im just so frustrated and sick of never getting to enjoy this car because of really hard to diagnose and really weird issues. I've yet to have an issue with this car where ANY of the usual suspects were the fault. its very defeating.

 

Those spots turned out to be motor oil. No brake fluid puddles or spots anywhere I could see. Underneath the ABS and Booster are bone dry both under the hood and on the concrete under the car.

 

Were both the front and rear/clutch circuit reservoirs empty? You'd surely have to be unlucky, these components you've replaced are they new or used? Unless NOS, the brake masters were NLA last I heard, so I'd want to be inspecting before install there.

If the fluid was disappearing with a drive around the block, and you think it's vacuum, just idling the car and exercising the brake and then the clutch should lead to the exact same outcome.

With only light yellow fluid available now and harder to see, a few paper towels at each and every connection, calipers, pump, under the reservoir, masters and slave should tell you if there's a leak you're somehow missing. I'm assuming if a clutch master was the cause over the last 5 months with multiple refills, your underfelt and carpet would be soaked and the interior would stink

 

Going back to your first post yesterday, #15:
Have you lost any fluid after you had replaced the master, or only before??? Talking about the fluid loss in this 'quick update', and in post #20, made it sound like you still have a brake fluid leak.

 

Very mysterious to lose a whole reservoirs worth of brake fluid and not see a puddle. The rest of the replies make sense to me - you can probably just press the clutch pedal over and over again, then check if your reservoir is empty, and repeat with the brake pedal. This should isolate one of two systems the leak is coming from.

A leak in your driver footwell will not produce a puddle under the car or in the engine bay, I would check that even if you replaced the hydraulics.

I just fixed a leak myself from the clutch softline to the slave cylinder hard line. It's held on to a bracket on the transmission under the TB elbow. There were a lot of wires and the subframe under mine that maybe it could catch a leak before it hit the floor? Though, probably not a whole reservoir's worth of fluid.

A leaking slave cylinder would probably leak into your bell housing, which wouldn't leak onto the ground. You might pull the slave and holder and feel around inside the bell housing for moistness.

Since you're having such a hard time pinning down the leak, I would not assume a new part is not leaking without checking first hand.