Sinking brake pedal while in gear.

[Incredibr0]

I took a drive to LA yesterday and my car started doing this. When driving and pressing the pedal, the pedal sinks to the floor slowly, kinda like the pressure i'm putting on it is being pressed out. When i lift my foot and press again immediately after the pedal is hard again. Luckily, having the manual lets me downshift to slow down and I rarely use much brake pressure anyway but this is something that's concerning. I'm thinking I may need to bleed the brakes to see if there's any air in the lines. The guy I bought it from said he recently replaced the master cylinder so I'm hoping it isn't that.

I checked the brake lines and their connections on the calipers and I didn't see any leaks so I'm hoping that bleeding them will fix the problem.

[Fleetw00d]

Where did he get the master cylinder? This is one of those things you don't want to skimp on. Air usually just makes the pedal spongy all the time. Bleeding them is a good idea; recommended order if I'm not mistaken is LR, RF, RR, LF. If you take the wheels off for better access, make sure you check that the pistons themselves aren't leaking either. Check as best you can above and around the fuel tank, the line for the RR runs over the top of the tank. Check that the master cylinder (I know it is new) isn't leaking along the push rod into the passenger compartment.

[deevergote]

A sinking pedal indicates an internal leak in the master cylinder. New parts are occasionally faulty, especially if they're lower-quality aftermarket parts.

I'd replace the master cylinder first. Either the previous owner installed a used or defective part, or he lied outright.

[F22Chris]

Check to see if your fluid level goes down too. Do you have drums in the back? If so gently pry up on the wheel cylinder boots. Replace if fluid comes out. Same goes for disc brakes, but with the piston boot.

When it sinks does it go all the way to the floor?

[deevergote]

I would imagine that if there's an external leak, it wouldn't be long before the pedal stops coming back up. There's not a whole lot of fluid in the system. (I recently had a ruptured line in my Cadillac... I probably got about 12 miles out of it before I had to purposely stall it out to stop it.)

[Incredibr0]

It doesn't sink all the way to the floor, it just starts out kinda squishy and then slowly sinks while maintaining pressure. it eventually gets to the floor but only upon stopping. Letting off the brake and then reapplying pressure re-stiffens the pedal. (so weird) I checked the reservoir yesterday and it looks murky kinda like miso soup. clear in some parts and miso-y in others. It was also filled All the way up when the max line is near the seam on the plastic. it was topped off.

[F22Chris]

Sounds like a master cylinder, but still bleed it first.

[deevergote]

That definitely sounds like an internal leak. The fluid should be clear as well, so chances are it somehow got contaminated. It would be wise to fully flush the lines before installing a new master cylinder. It's possible that the fluid had been contaminated when the previous owner changed the master cylinder, and that contaminated fluid killed it already.

[tommi]

brake booster line/hose good?

[Incredibr0]

Sounds like a master cylinder, but still bleed it first.

That definitely sounds like an internal leak. The fluid should be clear as well, so chances are it somehow got contaminated. It would be wise to fully flush the lines before installing a new master cylinder. It's possible that the fluid had been contaminated when the previous owner changed the master cylinder, and that contaminated fluid killed it already.

That kind of makes sense then why the fluid is different colors (murky and not) If he didnt flush it before installing the new one and the contaminants in it didnt get flushed. The MC is definitely new, its not anywhere as dirty as what's around it. Ill give the system a flush and then replace it.

brake booster line/hose good?

I'll take a look at it after work and see if it is, From my inspections yesterday everything looked good with that area but a second look wont hurt.

[SEgirl]

Mine has been doing this for a while now, too. They started sinking all the way to the floor, so figured it was the master cylinder since it was original (car had 300K+/- miles). Was done by Pep boys and a few months later, I noticed a lot of brake fluid coming out of the car. Took it back to them and they made it worse, so took it to the Honda dealer and they replaced the Master cylinder again. It still leaks, but not as bad, but the brakes are like yours. Will sink when you first press them, but after hitting them quickly a couple of times, they will "catch" and feel fine. ABS light has been on for several years, and code says it's the ABS pump.

It doesn't sink all the way to the floor, it just starts out kinda squishy and then slowly sinks while maintaining pressure. it eventually gets to the floor but only upon stopping. Letting off the brake and then reapplying pressure re-stiffens the pedal. (so weird) I checked the reservoir yesterday and it looks murky kinda like miso soup. clear in some parts and miso-y in others. It was also filled All the way up when the max line is near the seam on the plastic. it was topped off.