Last week I repaired all the small vacuum hoses under the hood which quite improved the power and slightly improved my brakes. I was driving around today and at one point floored it and drove fast about 300 yards. To my great surprise, the braking has completely changed after that: the pedal would kinda 'stick', in a sense like I would just have to press it slightly but that would provide a lot of breaking power. That was the opposite to the usual breaking pattern: I would have to press harder and harder to achieve more breaking.
Unfortunatelly, as soon as I would park the car for a while, it would go back to the old poor braking.
This has never happened before I fixed the hoses - flooring the car would not give me much more breaking, but I remember putting the AT into Power mode once when I was in a hurry and while I was driving aggressively to the appointment I had, the breaking was better than usual, but far not as good as today.
So it is not the pads, rotors, master cylinder or fluid that I thought were causing insufficient breaking - it's the vacuum. Where should I start looking for the root cause? Appreciate your help!

 

I'm not sure that I understand what your saying. I couldnt really make out what the problem is. explain a lil more clearly and maybe well be able to lead you in the right direction.

 

Your brake booster relies on vacuum. Fixing a vacuum leak must have improved its ability to function. That's my guess. Make sure the large vacuum hose feeding into it isn't cracked. I have a spare brake booster if yours is broken, FWI.

 

also check to make sure the hose going to the booster hasn't / isn't collapsed

 

The hoses look and feel Okay, as well as the valve. I had thought that the valve might have been stuck and only passed air at stronger vacuum than what's where with my usual laid back driving style. But it all looks good. Please, not a booster!

 

Brian:
Do you carry the vacuum booster hoses? The one from the intake manifold to the one-way valve on the firewall and the other from the valve to the brake booster? And also I could do with a PCV valve hose.

 

can't say I do.

 

1 way check valve in the brake booster line going bad?

 

Possible. It still holds vacuum, but I dunno how efficient.
I replaced the two hoses with generic 3/8" ID hose an hour ago and it slightly improved breaking.
What diameter is the PCV hose and hose between the throttle body and front valve cover? They are larger than what the local auto parts store sells under the name 'PCV hose'... I would have measured, but my caliper only reads decimal points of an inch...

 

I had a similar problem. My pedal would go almost to the floor before it starts braking. Sometimes, it would brake from a little push only. It turned out to be a sticky caliper. Check those out as well.

 

That has been checked long ago. All hydraulic parts of the breaks work fine. The vacuum sucks (oh, well, it always does, but in my case it does not suck good enough).