Brake fluid vanishing

[Scott Liggett]

The 58 Caddy I work on now has a disc brake setup. The rear brake resevoir goes empty in a week of daily driving. I can't find a leak and even replaced the rear wheel cylinders. I replaced the rear rubber lines a couple years ago as well.

The car was undercoated and if the brake line was cracked or rusted, it would be easy to find because fluid would eat away at the undercoating.

I thought it might be just evaporating from heat, but not likely as it is the stock setup.

So, I'm at a loss. Any ideas?

[milner351]

Leaking through the master cylinder seal into the booster?

[STINEY]

'58? I assume discs are swapped on or aftermarket....... residual valves?

The green sandrail in my avatar needs completely filled and bled every spring after winter storage. It just goes away, I've given up pondering on it and just accept its quirk. That doesn't help you much, but know you are not the only one.

I wonder if it evaporates out the tiny vent hole in the reservoir cap? Not in your case though, not in a week.

I'm strongly suspecting the vacuum booster is leaking it into the engine where it is burned and leaves via the tailpipes.

EDIT: John tree'd me again!

[Scott Liggett]

Leaking through the master cylinder seal into the booster?

Booster and master are new. I don't think that is the case because it's the front part of the reservoir that goes empty. The back stays full.

The kit is from ABS Brakes. We had a lot of trouble with it. Made them tow it down to Orange to fix issues. But, this isn't a cause.

[Scott Liggett]

'58? I assume discs are swapped on or aftermarket....... residual valves?

The green sandrail in my avatar needs completely filled and bled every spring after winter storage. It just goes away, I've given up pondering on it and just accept its quirk. That doesn't help you much, but know you are not the only one.

I wonder if it evaporates out the tiny vent hole in the reservoir cap? Not in your case though, not in a week.

I'm strongly suspecting the vacuum booster is leaking it into the engine where it is burned and leaves via the tailpipes.

EDIT: John tree'd me again!

It does have a residual check valve on the front and a adjustable proportioning valve on the rear.

[TheSilverBuick]

My Skylark does the same thing, except it only loses fluid if it sits. Doesn't seem to lose a drop when I'm driving it regularly, but if I leave it for a couple weeks the reservoir drains. I think I eventually found the problem with the proportioning/combination valve leaking. I'll know soon as the car is going to sit a while with the bumpers off and soon paint.

[anotheridiot]

if its not eating the undercoating, you can try to get some cheap black spray paint and spray around the fittings and wheel cylinders. brake fluid will strip paint, but it also will not let paint stick to it. its gotta be going somewhere and with no evidence its going straight down each time you step on the pedal.

[Scott Liggett]

Went down to look at it today. Saw that brake fluid was spilling down the sides of the master. I dried it off, then pumped the brakes a few times. More fluid on the sides. Looks like it's pushing past the lid and gasket. The bale that holds the lid down was extremely easy to remove. Too easy. So I tightened it up by bending it. I also noticed it looked as if the gasket was in the lid upside down.

Hopefully, it was something this stupid easy. But, at least, I got Gregg to open the hood of his car now and again before he it breaks down.

[anotheridiot]

It sounds too easy. are the brakes actually grabbing well in the back? maybe that adjustable proportioning valve needs to let some more pressure to the back brakes, like its causing too much resistance before the fluid actually gets to the cylinders. I mean the gasket and hold down sound right, but they are now putting little pos plastic reservoirs on master cylinders that sound like they might blow off in your case. It sounds like you have a pinched line, you know, step on the pedal and the fluid has no place to go, which is why I look at anything in that line.

[Barry Donovan]

the gasket rings a bell.

my chevelle did that locally. Try and find a poluyurethane or other type of gasket...

polymer chains expland distillates, and they don't stop on nitrile seals.
the old car, that is no doubt what it has.

seems to be a few episodes of that going around. makes fluid disappear, even power steering. I found the nitrile theory to be true. Nitrile rubber is the same stuff condoms and doctors gloves get, they want it to degrade...

we don't want that.

fancy chemicals today are stronger than what holds it sealed.

[JOES66FURY]

Went down to look at it today. Saw that brake fluid was spilling down the sides of the master. I dried it off, then pumped the brakes a few times. More fluid on the sides. Looks like it's pushing past the lid and gasket. The bale that holds the lid down was extremely easy to remove. Too easy. So I tightened it up by bending it. I also noticed it looked as if the gasket was in the lid upside down.

Hopefully, it was something this stupid easy. But, at least, I got Gregg to open the hood of his car now and again before he it breaks down.

Scott, I had this issue with my blue Ram, it was driving me nuts. after a month or so of driving it would be half way down. I checked everything and found that there was an issue with the cap. I replaced the cap and seal and the issue went away. Some times the fixes that are to easy are the hardest to accept.

[Scott Liggett]

Its an old school iron master with a metal top. The bale was so easy to remove. Just pull to a side with fingers. Most I've dealt with needed a screw driverr to pry it off. Anyways, Gregg will watch this week and see it is fixed. If not, we will continue the investiigation.