Brake fluid - slow loss. What to check?

[maverck]

1981 SC. 100k miles. New to us last year and as we have been sorting the car it has had common issues of a car that has been sitting like leaking fuel lines.

This summer about once a month or so I have had a spongy pedal and the reservoir is mostly empty. I fill it up and everything is fine for about a month and then it happens again.

There is no noticeable large leak. To that end what are the usual suspects I can check? Are there soft lines susceptible to leaking like the fuel lines did? Any other areas susceptible to a slow leak that would be less visible / hidden.

My plan would be to simply replace everything that is known to be problematic - which is in line with the plans for the vehicle overall - keep making it close to new over time.

[47silver]

I would not drive it until you repair it.
Start at the brake res hoses and mc connections
Check behind the floor boards see if the mc is leaking there.
Check every caliper.
Rubber hoses
And worst case the tunnel line. I had a vw brake line rupture in the tunnel from rust I think caused by not changing the fluid.

[GH85Carrera]

If you don’t see wet hoses or calipers, it is almost sure to be the Master Cylinder. Look carefully at the pedal area, and at the master cylinder carefully.

I would not drive it more than around the block until it is fixed.

[Mixed76]

A leaking caliper or calipers may not be super obvious, I've got one leaking that presents as slight staining inside the wheel and gummy brake dust on the rotor. On close inspection I can see wetness inside the caliper. No other symptoms- only saw it because I was adjusting the parking brake.

Stopped driving it obviously, a small leak can become a bigger one abruptly.

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[stownsen914]

I agree with the guidance to not drive it until you find and fix the leak. No amount of leaking brake fluid is acceptable.

If this were me, I'd hop in the car, step on the brake pedal with moderately firm pressure for 30 seconds, and then start looking your leak. Luckily a brake system is relatively well contained. Calipers, brake lines, reservoir, and master cylinder are the places I'd be looking. If you happened to bleed the brakes recently, check your bleeders.

[porsche930dude]

also check the hoses from the master to the resivoir

[icarp]

I bet you master is leaking into your brake booster

Ian

[Mr. Merk]

We had one last week. I used a mirror and saw a trail down the backside of the booster where the master cylinder had been leaking. The brake fluid eats the paint off, so a good time to repaint the booster while it's all apart. We also replaced the check valve on the booster just in case it was compromised by brake fluid that made its way into the booster. I'd normally replace the booster but they are $$$$

[maverck]

Good excuse for me to start at all four corners. Rebuilt calipers, SS lines, and pads ordered from our host. Was planning to do this job anyways and this just prompted it along.

Furthermore will do a bunch or investigation around the booster to see if I can find anything. Thanks all for what to look for thus far! Any other ides keep them coming and I will report back on what I find.

[PeteKz]

Maverck: AAs long as you're doing ll that, carefully inspect the metal hard lines and consider replacing them too. I used the nickel-copper line to replace mine. The old ones looked in good condition from the outside, with minor surface rust, but I figured I might as well go with something that won't corrode and be done with it.

[$yncro]

Originally Posted by icarp

I bet you master is leaking into your brake booster

Ian

Yup.. had the same issue, found my MC leaking in the booster.
Replaced MC with new switches. Easy peasy.

[maverck]

Originally Posted by PeteKz

Maverck: AAs long as you're doing ll that, carefully inspect the metal hard lines and consider replacing them too. I used the nickel-copper line to replace mine. The old ones looked in good condition from the outside, with minor surface rust, but I figured I might as well go with something that won't corrode and be done with it.

I was wondering if I should do all of that as well, even if they look ok.

[Jeff Burger]

Originally Posted by icarp

I bet you master is leaking into your brake booster

Ian

+1 The fluid doesn't just disappear. If you can't find any leaking fluid, it is likely your booster is leaking and the vacuum line to the booster is sucking it into your intake and burning it or it is sitting in the vacuum line. How do the brakes feel? when you start the car with you foot on the brake does the pedal sink at all as the booster starts getting vacuum?. good luck.

[Wrighteous]

I was in a similar situation, and determined a hard line was leaking. DR caliper had some schmutz around the connection. I'm rebuilding the calipers, so I'll just make new hard lines.

Jeff's point about the booster sucking fluid has me nervous... there was a bunch of crud in the vacuum line that feeds the booster. Any bright ideas on how to clean all that out? I figure liberal brake cleaner and a pipe cleaner but I'm open to suggestions. Thanks

[PeteKz]

If you are making new hard lines from scratch consider using the nickel-copper ones. I did, and love them. They won't rust, and the Ni-Cu alloy works easily and doesn't split when you form the flares. I liked it enough that I went ahead and replaced all the hard lines.