3000 miles back and January 2020 I did the usual brake job. Normal Textar pads. Rotors measure good and solid bleeding and PMB seating method. After that I had great brakes all the way up to now. With no warning pedal goes about 50% travel to the floor but still had weak braking. Found the reservoir empty to the bottom of the container. No sign of leaking fluid on the car, wheels, or garage floor. Filled it up and a short test drive brakes are back to normal. No loss of fluid.

Where did all the fluid go? If the master cylinder or a caliper started leaking wouldn’t there be evidence of fluid on the ground ?

with the MC its often under the wooden board protecting the brakes
most likely you will find the fluid there... puddled up in your pedal bushings

remove the rubber cap over the MC end, and see if it is wet under that rubber cover.

the pedal cluster base is like a little tray that will hold lots of fluid... plus the floor area is lower there that creates the pooling of fluids

Ah aha. I always thought the fluid would just runoff the master cylinder to the ground.

unfortunately no...
it needs to melt off all of your floorboard paint and pedal cluster paint before it gets to the ground.

ps. if you still have stock nylon bushings in the pedal cluster
they often swell and get destroyed too.

if you have upgraded brass bushings then they can be reused.

brant

Depends on where the point of failure is. If it's at the actuation piston where the pedal assembly pushes in it's inside the cluster. Clean it out as soon as you can as it will eact paint and cause corrosion.

What is the best cleaner for brake fluid?

water

I always use aerosol brake cleaner

It’s a little rough on paint but I figure that contaminated paint was already damaged or loose

Then a good paint prep cleaner before primer

This is not a 914 Reservoir but it works for illustrative purposes
Is normal fluid level at the top blue line mold seam or down at the clamp red line. Just can’t find that fluid level anywhere online or in the books. Thanks.

I don't know if this is right, but I always topped it off till the fluid hit the bottom of the filter screen.