Subaru Braking Issue When A/C is on

[BD_Shelton]

I drive a 1999 Subaru Legacy Outback, and I’m having an issue that’s puzzling me. When I run the A/C, I have to press the brake pedal much harder than usual in order to stop the car. However, when the A/C is off, the brake pedal is easier to press. Does this sound like a possible vacuum leak? I did a search on Google, and that seems to be the general consensus.

I’m pretty sure it’s not an issue with the brakes themselves, as they seem to function normally when the A/C is off. Also, I had all four brake pads replaced 6,000 miles ago, and the rotors were turned as well. I also had the brake fluid replaced at that time.

Topic

[David]

It does sound like a vacuum leak or your idle control valve.

[college man]

do the brake symptoms happen if you move the selector
switch to different vent modes without the ac? check the
brake booster vacuum line and fitting.

[Bill]

Does the engine idle a lot faster when the A/C is on?

[Konrad]

It does sound like insufficient vacuum at the booster. Shut the engine off and press the brake pedal. Is it really stiff, or do you hear a whoosh from the booster and the pedal feels ‘assisted’? If the pedal is assisted, try the same thing again but wait a couple minutes before pressing the pedal. Did the vacuum leak down and the pedal is now stiff? It may be a vacuum leak, or the vacuum check valve between the booster and engine.

[BD_Shelton]

Okay, I pressed the brake pedal and heard the “whoosh” sound. The pedal stiffened up after a few pumps. I left my foot on the brake, started the car up, and could feel the pedal depress. I turned the A/C on, and the RPMs didn’t fluctuate.

The weird thing is the pedal didn’t feel stiff this time when I reached my first stop sign. So it’s either an intermittent problem, or something else is at work here. The only other thing I can think of is that I was carrying 3 other passengers at the time when the pedal felt heavy. I heard added weight can make it harder to stop a vehicle, so I’m wondering if this had something to do with it. The car stopped fine, it just required more pressure than usual to press the brake pedal. I just began pressing it sooner to compensate.

[Bill]

Yes, the extra weight in the car could cause it to be harder to stop especially if you normally drive alone and are used to it.

[Author]

Replies