I keep my 2016 Ford F150 Lariat parked in my garage over night and every morning I start it up to go buy a newspaper. Just an old habit. The truck has the Start-Stop button for the engine. Most times I place my right foot on the brake pedal and the pedal will depress an inch or two. Then I hit the Start-Stop button. But once in a while I place my foot on the brake pedal and the pedal does not move and feels hard. Truck starts fine, and no braking problems. I was wondering if anyone can explain why the brake pedal will depress sometimes and other times it’s hard as a rock?
Power brakes run on engine vacuum. After you shut it off, the vacuum isn’t generated anymore, but any residual vacuum will continue to help you press the pedal. Once the vacuum is gone, the pedal gets much harder to press because now your leg is doing the work that the power assist was doing before.
If I had to guess as to why sometimes there’s residual vacuum in the morning and sometimes there’s not, I would hazard that perhaps on the mornings when the pedal is hard, when you shut down the day before you pressed the brake pedal again while getting out of the truck, which dumped the residual vacuum.
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Thanks, that’s a logical answer. I have to drive somewhere later today and I’ll experiment and post back my findings.
@Thornback Why did you change your screen name?
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I don’t remember changing my screen name. What do you say it was before?
Was it not ’ Patgurr’ ?
I just checked my records and see I changed my password back in July. I probably also changed my screen name at that time. I’m not being evasive, I just don’t remember. But I have used patgurr before. Why do you ask?
does this affect their brake pedal feel?
Shadowfax – I tried several different ways to start - stop the engine but could not duplicate a hard brake pedal. But if I stop the engine, then pump the brake pedal 4 or 5 times that makes a hard pedal. Then the next time I start the engine the brake pedal will not depress.
The first sentence is normal. The second isn’t.
Shadow may be right about the cause. Try pumping the brakes with the engine off and then starting the engine with your foot pushing the pedal. The pedal should harden and then, when you turn the engine on, sink some. If it doesn’t sink, the booster is failing.
But the cause could be as simple as a failing/sticking check valve, which is what I suspect. The check valve will be at the booster, on the vacuum line that goes from the engine to the booster canister. If it’s plugged up, or intermittently plugging up, that’d cause your symptoms. The vacuum won’t be able to pull the booster’s diaphragm if the check valve is plugged up.
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I have replaced several brake boosters that leak down over night. People didn’t notice this problem years ago but with push button start the brake must be applied to start the engine and some people can’t push the pedal without power assist, they have the car towed in.
If your truck is under warranty you should be able to get this repaired but it may be difficult for them to duplicate the problem.
@volvo_v70 nobody changed their username. We fixed an anomaly whereby the names people signed up with were substituted with often the first part of their email addresses. This led to us seeing lots of people’s names. Once we fixed the setting, the names people chose started showing up. And one by-product is that now @the_same_mountainbike isn’t missing his final e anymore. 
Ah so, that explains it. I automatically log into many websites with various names, thus I don’t pay attention, and so when questioned I just couldn’t remember changing my screen name. Thanks cdaquila, I thought age might have caught up with my pea brain.
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I trade every two years so In a couple of months I’ll be ordering a new 2018 F-150 so I’ll not bother the dealer with my small problem. Thanks to all who replied to my query.
Good Greif ! 
Don’t be surprised. My present F-150 is my 11th new truck – 10 were all special ordered and purchased from the same salesman. I retired 22 years ago and swore then I would not let my vehicle grow old like me 
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To me it’s nobody’s business what I call myself unless it’s a conflict-creator. Even if I change it.
I’m sure. however, that your explanation is appreciated. 