How to diagnose brake master cylinder vs. brake servo/booster?

[Andre]

Got a bit of an issue with the brakes on my Celica I’m having trouble diagnosing, any help appreciated. It’s had spongy brake pedal in the past inspection advisories from a few years ago and no ones bothered to fix it I guess.

– brake pedal feels solid until you start the engine
– holding the pedal and starting the engine, it goes almost to the floor
– it’s drivable, has good initial bite, but needs more pedal force for heavier or longer braking and the pedal ends up near the floor
– bled brakes using two man method in correct order
– bled brakes using one man bleeder in random order
– if I disconnect the vacuum hose going to the booster/servo thing, the pedal remains solid and does not go to the floor

Obviously checked for leaks and there’s nothing, calipers are ok, sliders are free and pistons compress back in fine.

Does this sound like the servo? Is that common?

Topic

[Richard Kirshy]

If there is no sign(s) of leakage … I hope you did check the vacuum hose connected to the booster.
You can test the brake booster with a manual vacuum pump to make sure it isn’t leaking.
But more likely, there is probably a bad seal inside the master cylinder allowing brake fluid to bypass.

[Andre]

Hey thanks for the reply.

I have a vacuum pump, how much vacuum should I apply to it? Assume I should observe that it doesn’t drop? Do I need to press the brake pedal,

[Richard Kirshy]

10 – 15 in of vacuum should be enough to check it for any leaks and no, no need to press the brake pedal.
Keep in mind if the was a leak in the brake booster, most likely you would have a harder brake pedal due to lack of vacuum.
One other item to check would be the check valve on the boosters vacuum hose.

• This reply was modified 4 years, 4 months ago by Richard Kirshy.

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