Failing brake master or power booster?

Hi folks - want to to make sure I’m approaching this the right way and going to to fix the actual problem.

Issue: brake get soft and warning light briefly flashes.

Condition and testing;

Drive the car around a bit and then shut it off and pump the brake pedal. Pedal gets firm with car off. Turn the car back on and the pedal stinks little bit, but not all the way to the floor…

Master cylinder is almost completely full of brake fluid.

That sounds like master cylinder rather than booster to me but wanted to ask your opinion.


That’s a hydraulic issue so the master cylinder.

My car currently is waiting for a new booster to be installed. The pedal is VERY firm and produces little stopping power. Also the engine idles roughly for a few seconds when the brake pedal is pressed.

When your brake pedal gets soft (or spongy) it generally means the brake fluid is contaminated and needs to be flushed then replaced.

But check for leaks in the rest of the brake system.

If you think there is a problem with master cylinder, disconnect brake master cylinder from the brake booster, then check to see if any fluid in the cavity between master cylinder and where it slots into brake booster. (There should be a weep hole where fluid drains out here, it will show fluid leak below master cylinder) If there is any sign of wetness, master cylinder is leaking.

That is how you test the brake booster, booster is doing what it is supposed too, so booster is good.

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