Mystery Brake Fluid Leak
 

Driving to work the other day my brake light came on while driving up hill. Once at work I noticed my brake fluid was getting low. Driving home I noticed the first pump of the pedal felt a bit squidgy, the second tightened up. I'm suspecting a brake fluid leak. Over the past year I replaced the clutch slave and master as well as the rubber hose. I also flushed the fluid with my Motive bleeder. After work today I jacked the rear up and looked at the brake hoses and clutch slave, no leaks. I took the front passenger side wheel off to inspect the master cylinder and brake hose. Pictures are attatched. The master didnt have any fluid on it. The steering rack is leaking and I assume the fluid in the pan is from that. I didnt take the drivers wheel off but inspected the brake hose and no leaks. Lastly I took the frunk carpet out and looked at the ABS booster pump, no leaks. Are there any other rubber brake hoses to inspect? Ill take all the pans off this weekend for a better look but I'm stressing bit after seeing the $1500 master cylinder price tag. I dont see any puddles on the ground other than the usual one from the steering rack. No weird smells driving it either. Any help would be appreciated.


https://cimg6.ibsrv.net/gimg/rennlis...8738a2f274.jpg

https://cimg7.ibsrv.net/gimg/rennlis...0179d19401.jpg

Could this be a residual of a spill during the brake fluid flush? When you bled the brakes, did you remember to clamp off the brake fluid reservoir vent hose?

Lastly, how low was the fluid level in the reservoir? Its been my experience that the light will come on if the level is not at or near the Max line.

There are rubber from reservoir to master. 3 of them 2 for brake 1 for clutch
Only other rubber are for each wheel caliper. Those in good shape? You check you pistons on calipers?

For that price I'd be putting the 993 master on it. An oem try part is $350. You will need a 993 booster, 993 bracket, and to form 2 lines from the master to the abs unit but you'll get an upgrade for less $$. I didn't realize the 964 mc was so much $$$$

The fluid level is about half a centimeter above the MIN line. The car has performed flawlessly for 9 months and do not suspect the flush was the issue. Im stumped because I cant find an obvious puddle of fluid. There is no fluid in the wheels from leaky calipers, no fluid at the brake hoses. No fluid in the frunk floor by the ABS pump. Ill take the belly pans off this weekend, dig in further and take some pictures. My buddy might have a 993 master/booster assembly laying around. Does the 993 master work well with the stock 92 C2 brakes? I'm not planning on upgrading to big reds anytime soon.

Would want others to weigh in but it should work OK. You'll get a 'firmer' pedal with less stroke due to the larger bore size.

Its a 'mostly' bolt on mod, you can see my build thread... there are a few tweaks but its not a huge deal.

The system is designed so that the pickup for the clutch master is ABOVE the brake pickup. If you were running low on fluid, you would then experience funky clutch action before the brakes act up.

How does the clutch feel to you?

I am stumped as to where the fluid went. Have the car up on stands in the garage, took the front belly pan off and the master cylinder, booster, and hard lines are bone dry. The calipers arent leaking, the hoses arent either. I got my hand and mirror up to the clutch and the hose is dry, the slave is dry. Took the floor board out by the pedals and the clutch master is bone dry as well. Ill pick up some ATE Gold and bleed the whole thing tomorrow and see whats up.

Took all the under trays off. There is no sign of a leak on anything. You can see the picture of the fluid level in the reservoir. It was at the MAX line. You can see the master cylinder is dry as well as the ABS pump. I removed the clutch inspection rubber cover and there is no fluid leaking out of the slave. Is it possible the fluid is leaking into the booster from the back side of the master cylinder? Is it possible to lose that much fluid just from brake pad wear?

https://cimg0.ibsrv.net/gimg/rennlis...24f346fd61.jpg

https://cimg9.ibsrv.net/gimg/rennlis...70b43a421a.jpg

https://cimg1.ibsrv.net/gimg/rennlis...23cc968d90.jpg

https://cimg2.ibsrv.net/gimg/rennlis...bf45f3dec0.jpg

this is bizarre. the brake system is pretty straight forward and you've checked everything mostly
1. starting from the back, check each caliper, check rear rubber lines
2. on the front, check each caliper, check each rubber line
3. in the frunk check abs lines
4. inside the car check under the floor board to the clutch master
5. on the trans check the slave / rubber hose - you aren't getting any funky clutch action are you??
6. check the fluid reservoir and lines to the masters
7. Last culprit is the brake master... if it's my car i'm pulling it. Yes, it could leak inside the booster - but i'm unsure if the design would contain all that fluid... time to check!!

Agreed, very bizarre. If this is a C4, also check the two PDAS slaves on the transmission.

has it slowly disappeared?
could you have possibly backed all the callipers off when doing the brake bleed, leaving room for the fluid to refill the callipers (upon use)?

It appears to have slowly disappeared. Is it feasible to lose such a large volume of fluid? These cars have large (4) piston calipers compared to what I'm used to working on so maybe its possible. Its been a busy week have haven't had time yet to bleed everything. Picking up some fluid today on the way home and will try to get to it this weekend or sooner.

did you physically push back the pistons at any point in time?

Never. Just used my Motive Bleeder to bleed it. The pads looked fairly new when bled it a year ago. It was a few months after I bought it. The rear pistons look to be extended about 1/8" now.

you would not lose that much fluid through piston movement. something is leaking, somewhere.