I'm definately losing brake fluid. I had the lines from the rear splitter to the wheels redone a year ago and all that is dry. None of the lines in the engine compartment are wet either. I don't see any spots of DOT 3 anywhere!!! Any ideas for troubleshooting?

Master cylinder leaking into brake booster?

Will it drip out of the booster?

It might not. I do not know if there is any sort of weep hole on that particular booster; if not it would probably collect between the M/C and the booster diaphragm. Also, if the diaphragm is ruptured, it can suck fluid into the booster's vacuum line.

 

I should have asked: What makes you think that you are losing fluid? Was there a sudden drop in fluid level? If not, is it possible that it is just brake wear causing the pads and/or shoes to extend further out and needing extra fluid to keep them out?

I keep on having to add brake fluid. More than due to normal wear. Gotta check the bottom of the booster Monday and will report findings.

Another thing I should have asked is what car this on. I am assuming a Legacy based on username. Rear disc or drum brake? (Drum brakes can hide leaks.) ABS? (Does not sound like this is a concern if all engine compartment lines/fittings look dry.) Does the brake pedal "feel" normal?

98 leg O. With 4 wheel disc and ABS. Every now and then the pedal feels a little mushy. Not all the time.

Disc calipers don't hide piston leaks very well, you checked the engine compartment plumbing for leaks, and the rear junction/splitter(/proportioning valve?). If you have looked at the lines to the wheels and the plumbing going back to the rear splitter, I can't think of much other than the master cylinder. (I am not sure if the ABS pump can leak internally; just haven't paid attention.) Occasionally spongy brake pedal might point at the MC.

 

I hope others chime-in in case I am overlooking something. Otherwise, I might look into partially separating the MC from the booster to see if any fluid leaks out. Just for grins, might pull the vacuum line loose from the booster and check for wetness in the line (doubt that it will be, but easily enough to check anyway).

Are you sure the calipers are not leaking. The better halfs 98 Legacy had a leaking front caliper, but it would only leak when it was below freezing.

Replaced both front calipers a year ago ---not those. Gotta get another good dry day. Just snowed today and tracking wetness is tough.

Engine running stand on the brake pedal repeatedly for about 20 seconds at a time for a few minutes. If you have a leak, you will feel the pedal will sink, and after repeated application, any fluid leakage should be evident on the ground or someplace on the car. Of course, having a dry place to work helps.

I'm definately losing brake fluid. I had the lines from the rear splitter to the wheels redone a year ago and all that is dry. None of the lines in the engine compartment are wet either. I don't see any spots of DOT 3 anywhere!!! Any ideas for troubleshooting?

 

look under the seat

look under the seat

 

Which one? Might it have rolled under? Is that where the brake line goes when It's not under the vehicle?

Your master cylinder is leaking into the Power Brake booster while the car is running. Check your master cylinder at the piston seal.

Detach master cylinder from booster? There are no external leaks in that area. Is it a replaceable seal?

the seal is leaking due to engine vacume. It is leaking into the booster and the engine is burning it. You never rebuild a MC you just replace it.

WOW!! So I just buy\install a new master cylinder and I'm good? How much damage does burning brake fluid do to the engine etc.? Who knows how long that's been goin on?

No damage to engine. That is where you look when you have checked everything else. Nothing left after that.