brakes work fine with appropriate brake boost EXCEPT when car is creeping at low speed and is at idle. Suddenly it feels as though there is no assisted braking and a lot of effort is required to stop the car. During normal driving car brakes fine.

No CEL
no erratic idle
no recent brake fluid change
pads/rotors are fine


sounds like a Vac leak somewhere.

any ideas on how to pinpoint or where to smoke the system out from?


tia!

 

With a vacuum assisted brake booster when you press down on the brake pedal a a valve closes off atmospheric pressure to one side of the brake booster piston. Continued pressing of the brake pedal then should have a line to the intake and vacuum opening up that supplies vacuum/low pressure to the other side of the brake booster piston. The atmospheric pressure on one side of the piston and vacuum (low pressure) on the other side provides the assist.

However, given the above, I'm not sure the circumstances that would have the brake booster working fine at higher speed.

Well, besides a vacuum leak perhaps. But one that is present only at low speed and not at high speed?

It is possible. A small vacuum leak that at low speed is significant but at higher speed -- higher engine speed -- is less significant.

But with a vacuum leak at low engine speed I think there'd be a CEL with a mixture error code.

If you can connect an OBD2 code reader/data logger to the OBD2 connector and monitor intake air pressure/vacuum in real time you might see a reading that could provide you with some indication there is a leak.

Beyond that I've never had to trouble shoot a malfunctioning brake booster system so that's all I have.