Vanishing Brake Fluid

[Highlander]

My youngest daughter now has the '85 sunroof car and is using it for commuting to work and back, a couple of weeks ago, it ate the timing belt (I had no idea of the age of it), Dragged it home with the truck (15 miles up and down hills) with my eldest driving the truck (she had no experience at doing this), she didn't slow down at the tops of the hills so that the terc's brakes could handle the weight going down the hill.
Long story short, got the timing belt changed, and the brake light was on steady, bled the brakes, (Black fluid from the rears!) and all was good.
The next weekend, her brake light was back on due to low brake fluid, topped it up, checked all of the wheel cylinders and calipers, no leaks. brakes still good after a touchup on bleeding. She came back this weekend and the brake light came back on, low fluid level again. lifted the car, rolled underneath, no leaks, no brake line damage, dry wheels (rear), dry calipers, no fluid in the vacuum booster line.
So I'm stumped as to where the brake fluid is going. It seems to lose it when it sits, not driving (it sits during the week), but there is no puddle anywhere.
Any thoughts?

[irowiki]

Did it cook the cap so it's leaking out of that (evaporating? does brake fluid even evaporate?)

[marlinh]

I have never heard of brake fluid evaporating. Check around the master cylinder. The gaskets that seal the reservoir can leak. The master between the brake booster also. It may not make it to the ground to show a puddle. But be damp.

Did you pull the drums off to check the wheel cylinders? Are the brakes spongy at all or firm?

[ARCHINSTL]

The master between the brake booster also. It may not make it to the ground to show a puddle.

This exact condition happened on my '88 Dodge/Mitsu p'up a few years back. Of course, it was the last thing I checked...
Tom M.

[Highlander]

I have never heard of brake fluid evaporating. Check around the master cylinder. The gaskets that seal the reservoir can leak. The master between the brake booster also. It may not make it to the ground to show a puddle. But be damp.

Did you pull the drums off to check the wheel cylinders? Are the brakes spongy at all or firm?

Brake fluid won't evaporate, it's made to not boil until around 450°F- I always use Castrol GTLMA so its boiling point is a bit higher depending on how much water its absorbed.
There are no brake fluid tracks on the inside of the rear wheels, if it was leaking at the wheels cylinders, it would come out onto the tires, BTDT.
I checked all around the master and inside of the engine compartment, no signs of brake fluid. brakes are nice and firm until the level gets low.
I know it has to be going somewhere, I'll check the face of the booster next time.
It might be filling the booster housing, I've seen that before with my Ford 1 ton...
At least I have some spares

[Petros]

check under the front carpets at the driver's feet, if the seals in the master are leaking it will leak inside the firewall, run down under the carpets to the floor. Also look for streaks runnign down the fire wall inside the driver's compartment, it will be under the liner and insinuation. you will not see it until the carpets float on it.

It is also possible it might be running down the outside of the firewall under the master cylinder, but that should be easy to see.

[simon84]

Can I jump in on this thread?
I am losing brake fluid from the master cylinder and can see that it is dripping down from the reservoir.
Is it a matter of replacing gasket seals under the reservoir as Marlinh mentions above?
Are those available? I checked and RockAuto has the master cylinder with reservoir attached. So that could be one approach but I'd be interested in a simpler solution.
Thanks in advance for any tips...

[dlb]

yeah simon, there is a thick rubber washer (or two? can't remember) that goes between the reservoir and the body of the master cylinder. not sure if or where you could get that seal but if you found a match, that's all you should need to replace. i think you would have to remove the master cylinder to bench bleed it before reinstalling it, though.

[Petros]

try pushing the reservoir down (wiggle it and push down hard) into the master first, to see if that will stop the leak. It is only held in place as I recall by the friction between the reservoir and the rubber o-rings on the master cylinder. Otherwise you will need to replace the 0-rings as suggest above.

good luck.

[splatterdog]

As far as fluid loss with NO sign off leakage- I would pull the master(lines still attached) to be sure the rear seal is good. Then carefully "dipstick" the booster, if possible, with a zip tie to be absolutely sure there's no fluid present. Some cars won't show piston leakage dripping down the booster face. Usually its real obvious..

Simon- There should be grommets between the master and reservoir. That's how every plastic res has been attached that I've seen. You just carefully pry the res up and then just push it back in. With that being said, I would still replace the master. How much better are the seals inside? Is it decades old like most of ours? I haven't had to replace one yet on any of mine. In fact, I can't really remember ever replacing a Toyota master, and I've been wrenching since T4's were still available new. But, they all have to wear out sometime right?

I don't ever recall more than one or two reservoir gaskets leaking, even on domestics, so maybe a warning sign? At minimum you will probably need a full rebuild kit just to get a grommet. I can see your local Toy parts guy rolling his eyes and/or saying what's a reservoir grommet???

[simon84]

I pulled out the reservoir and cleaned things up and put it back. So far it's not leaking but I suspect that with driving vibration that it may lose some fluid. We'll see when I get it back on the road in a couple of days. I just did a bunch of front end work, a brake hose and put in a lift (spacer in the front and pathfinder springs in the back).

Thanks for the tips.