Brake Issue - Soft Pedal?
 

I noticed recently that my brake pedal is soft. I'm hoping to get some insight before discussing it (again) with the dealer.

There is no visible leakage and the master cylinder reservoir maintains the proper level of fluid. The brakes stop well with minor pressure on the pedal. A month ago the dealer did a 4-wheel brake job; mileage is just over 50,000. Recently I observed that if I'm at a stop and I add pressure to the pedal, it will push down almost, or maybe all the way, to the floor. It does not seem to reduce the braking power at all. I honestly don't know whether it was always like that and I never noticed, or it began after the brake job last month.

Today I had the dealer diagnose the issue, replace the brake fluid, and bleed the brakes. They were satisfied, but I think the exact same issue remains.

First, does this behavior seem abnormal? My only point of reference is my old 1992 F150 and older cars without the extra electronics involved, all of them having tight, firm pedals.

Second, the service manual talks about inspecting for "master cylinder leakage into the brake booster" and "evidence of brake fluid in the vacuum tube." Can these even be done without removing the components and replacing them?

Thanks for your insights.

It sounds like a common and typical brake pedal operation.

Because of changes in vacuum assist the pedal can creep when at idle.

You can say you've never noticed this before but I can promise you every vehicle I've driven with vacuum boost brakes will exude this symptom.

It it is common to you, and only happens while stopped at idle -chances are it's a result of increased vacuum.
The thing concerning is that you say you can push the pedal to the floor. Is that an observed foot on the floor or is this a "it moved more than I think it should and is probably getting lose to end of travel now".

Thank you, 16IngotFX4. You are correct, the pedal feels like it travels too far, but I haven't measured it. It probably does not hit the floor, just seems like I'm getting close to it. I'll play with it a bit and see how it works out -- stopping power and holding power remain excellent.

Definitely pay attention that it isn't more than I suggest -but I would state that is is uncommon for the pedal not to move than it is TO move.

When you apply pressure from a stop, don't push too hard but then stop moving your foot. If you creep foward I'd assume you are no longer applying enough braking to hold back the vehicle (meaning you're pushing hard but the fluid is not providing pressure to the wheels).

I routinely feel the pedal become soft about the time I stop, or something like that. I mean I don't feel out of place that the pedal moves from time to time but braking is unchanged.

Good luck! Let is know if this sounds like what you continue experiencing.

There was a recall on select models and years for a master cylinder leak. May want to search your VIN just to be safe.

Oh shoot -that's a great reminder.

Duh, I just read the old piece of mail the other day that I received a couple months ago!

I don't know a lot of the details but I think it only specified the 3.5 eco's? I just figured it was worth a mention as it's easy enough to check your VIN for the recall.

Mine wasn't recalled, but I did test it a bit on the way home. The pedal only gets soft when sitting at idle. Fast stops, panic stops, holding steady (even when the pedal drops down at idle) all seem to be A-OK. I think 16IngotFX4 has the answer.

Thanks!

First time for everything...

Hydraulic cylinders can maintain pressure while also leaking as the piston travels down the bore. And the master cylinders typically have three seals inside, two to create pressure in the brake lines and one to keep all fluid in the master cylinder, so you might not have an external leak. It's not uncommon to damage a pressure seal when bleeding brakes because the seals travel over areas of the cylinder that haven't seen any action and can get corroded and rough. Very common in old cars when people try to get them running again.

Anyway, it kind of sounds like your pressure seals are starting to fail. There's a bunch on the internet about it. Here's a decent one.

https://www.brakeandfrontend.com/bas...ster-cylinder/

"The most common problems that occur in the master cylinder is wear in the piston bore and piston seal failure. The classic symptom of a failing master cylinder is a brake pedal that slowly sinks while pressure is held against the pedal. The cure is to replace the master cylinder."