While we were on our trip (Gilmore in the middle) our V70T5 performed well but turned over 150,000 miles.
We did have several disturbing braking incidents and signs of a leaking booster.
While on the highway my tinnitis was so bad that I could not hear the hissing/sighing
coming from around the brake pedal, but in town I could. It stops when I step on the brake.
The brakes generally work very well but occasionally, especially after a while at highway speeds,
when I want to slow down or disengage the cruise control, the brake pedal is quite "hard".
Pressing on it firmly seems to reactivate the booster and things are then normal, with the
brakes quite sensitive. So while it is initially scary, once I press the brake hard,
it seems to be OK.
Are both this symptom and the hissing/sighing we hear symptoms of a booster on the way out?
Or are there TWO problems developing?

I'm interested in the solution, if any.

Bill

I ordered a booster from FCPGroton (with a lot of other stuff) apparently after closing Friday.
I had had a problem with the "ejector valve" hooked to a tube off the end of the intake manifold
on a trip a year and a half ago. Being on the road at the time I glued it together with
Krazy Glue and then taped it with white duct tape.
I noticed that the valve was apart and the duct tape had stretched with the underhood heat.
I suspect that the hard push on the brake caused it to suck back together.
I have reglued it and also ordered another valve. Together with the new booster
I am pretty sure that will resolve this particular problem.

Is that ejector valve screwing up your vacuum?
maybe you just need a new valve and the booster is fine?
If you loose vacuum the booster will quit. I'm struggling with
an intermittant vacuum problem here.

Bill

I can hear the booster sucking and sighing around the actuating rod to the brake pedal.
I know it is headed south.
When the ejector valve pops apart, vacuum to the brakes is lost. It was taped in such a way that
sufficient vacuum would suck it back together. Its innards are fine.

Somewhere I read about some little one-way valve that can kill the booster vacuum. Maybe that's the noisemaker?

It would not be making sucking and sighing noises around the brake pedal actuating rod.
I've had failing boosters before and this is the classic symptom of a diaphragm on the way out.
(The brakes still work fine.)

Just as a point of inquiry, when was the last time you had your brake fluid flushed? This should be done every 2 years MAX and it's pretty cheap insurance that nothing else is malfunctioning in your braking system.

I'm not sure about your hissing/sighing noises though.

--
1998 V70 AWD->FWD Turbo 200k+