Wife's car. Brake has good pedal, but every once in a while pedal drops to the floor. Happens more often when she is waiting at a traffic light.
I went over all the brakes today. Checked the wheel cylinders, and everything else. No leaks. I bled the front brakes. Pads and shoes all good.
Bad Master Cylinder?
MC leaking into the booster?
Yeah, bad master cyl. It should only happen when you press pretty gently on the brake pedal for an extended period (like sitting at a light). If you mash the pedal, the high pressure will sort of balloon the seal and it will feel normal. Here's what the seal looks like - you can see how it will push out against the wall of the cylinder if there's a big pressure difference.
It's an internal seal, so there won't be any fluid physically leaking out that you'd be able to see. It just leaks between the high and low pressure sides of the seal.
Out of curiosity, how many miles are on it? I've only had this happen to cars with a ton of city miles, where the master cyl has seen a lot of wear.
Internal leak down in m/cylinder. To diagnose: place foot on pedal with engine running, just rest foot on pedal, do not apply pressure, if you apply pressure seals will flex outward and leaking will not occur. Pedal will sink to floor. Do not drive car, replace M/cylinder and flush brake fluid.
I diagnosed MGB with same symptoms and told customer to park until they could afford replacement. Sibling came home, went for a ride and ran into the back of another car.
JohnyHachi6 wrote: Out of curiosity, how many miles are on it? I've only had this happen to cars with a ton of city miles, where the master cyl has seen a lot of wear.
Just about 110k kms or roughly 70k miles. But it is 25 years old. Summer car only.
Yup, bad master. Classic symptoms.
Put a master cylinder in it, no more driving until that's done.
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