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Master cylinder or brake booster

Hi everyone,

I’ve got a car with frightening little brake pressure. I swapped out the rear calipers after a failed inspection, bled them as I’ve done many times in the past, and thought i was good to go.

With the car off, within 3 pumps, the pedal is rock hard. As soon as I start the car, the pedal goes almost all the way to the floor, but I have enough pedal to stop the car, but not on a dime. No amount of pumping brings the pressure back.

I should mention, the brakes were fine before changing the calipers

There are no external leaks and no air in the system, so please don’t waste time saying “you’ve got air in it” or “check for leaks” or “bleed them again.” I know it is not the lines, or calipers.

It’s either the booster or the master cylinder. But from what I’ve read, it’s not the booster. I was just looking for a second opinion based on the systems I’m having.

Thanks in advance

sorry. bleed them again, correctly and completely.

for this kind of bleeding i use a clear hose, and starting at RR, hook the hose up and let it flow into the reservoir. do this for each one. ensure the reservoir does not run empty.

i suspect you got air in the MC, (although you may not have caught it) and that is causing a problem.

have you changed the MC recently? or adjusted the brake pedal? sounds like the MC may be not getting full throw to bleed it fully.

did it, no bubbles. there was no opportunity for air to enter…reservoir was full, disconnected brake lines for 5 secs, which only lets fluid out, not air in.

master cylinder is original equipment.

after thinking about it, you say, “after three pumps the brake pedal is hard”

then when the car is turned on, the pedal mushes. sounds like the booster is not working.

what else did you do under the hood recently? mess with any vacuum lines? especially to the booster?

haven’t touched anything. Definitely not my first brake job. This is something coincidental or maybe not. I read somewhere letting the pedal go to the floor when bleeding was bad for the master cylinder. I’ve done it this way my whole life on many cars, never with a problem. But the car in question has 202,000 miles on it. Maybe it was just time. All i know is it’s not a typical brake component that is the problem, just wanted another vote for master cylinder.

If the booster is bad, the brake pedal would remain firm after starting the car. It is rock hard in 3 pumps now, but once i start it, the pedal dissappears.

Booster is good. I would look at the calipers, they may not be floating properly. After releasing the brakes, you should still feel some drag on the brakes when you turn the wheel. The drag should be on both sides of the rotor. Look very closely here, the problem could be in the caliper bushings.

I am hoping you have the rear calipers installed with the bleeders facing up. People do make that mistake and if so it’s air.

if the MC was bad i would think you would get a soft pedal. maybe hard for a while, then slowly mushing down.

if it has to do with the engine turning on, the vacuum is changing in the booster.

the calipers were installed correctly. New guides were installed and greased. I’ve bled systems that are nothing but air, and after enough bleeding, they work properly. I am sure there are no external leaks or air. The system was sound before, and only the caliper lines were disconnected. Has anyone ever experienced a failed master cylinder? Or booster? What are the symptoms?

Yep, you have the symptoms of a failed master cylinder, but the coincidence of it going bad at the exact moment you worked on the calipers bothers me. Did you replace the crush washers on the brake lines at the calipers (if they are present)? What kind of car are we talking about?

It bothers me too. I have accidentally fried a stator on a moped, but that’s it, and that was a result of my not paying enough attention to what I was doing. But working on brakes is old hat to most D.I.Y’s.

Does it have anything to do with the brake pedal going full travel? By that I mean, letting the pedal go to it’s lowest possible position when the bleeder is open.

The car is a 93 mitsubishi eclipse. I have never had to change a bearing, head gasket, nothing. I had to put in the first replacement strut this weekend. Aside from brakes, tires, and exhaust, and a little body work, it hasn’t needed much. And with 200,000+ miles, still getting over 30mpg, I can’t let it go.

And yes, I replaced all crush washers (2 per caliper)

If this vehicle is more than ten years old, I’d suspect the brake hose(s) on the rear of having internal deterioration. There is no “blow out” or “fix” for that, other than changing the hose(s). Also, carefully inspect the full length of the brake lines, for kinks, dents,n crushes, or twists.

the fluid coming out of the rear bleeder is transparent and contains no particles or sediment