disappearing brakes?

Hmmm,

I have had a similar problem in the past, but in my case, I had found that a wheel bearing was collapsing with excessive wear, that when right hand turn were being negotiated, immediately, after, when the brakes were applied; the brake pedal would travel straight to the floor.

Of course, the next pedal pressed, would bring the pedal pressure up again
untill the next time a right hand turn was made.

In this instance, the right side wheel bearing was failling. With excessive play, turning right, meant the wheel would rotate out of true on its axis, causing the caliper pistons to be pushed in. Hence, the first brake application there after, would mean the pedal travelling straight to the floor, filling the piston area, dissipated earlier in the turning. After the filling, the next pedal pressed would have normal pedal.

So, after all that, check on your wheel bearings.

As far as the vacuum system is concerned, it is there to provide servo assistance to braking. If the vacuum servo fails, the brake pedal gets heavier, not collapsing.

Keep in touch,

Regards,