Crazy brake problem

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Well-known member
Mar 29, 2012
Berry, Alabama
Was in my 99 F150 last Friday. Stopped for about 15 minutes to get some power steering fluid for the wife’s Jeep, came back out, when I got in to start it, the brake pedal nearly hit the floor. Let off, one pump and it was back to normal. Went down the road a couple of miles through a red light or two, brakes still seemed soft, but not as low. Checked master cylinder, it was full, didn’t see any leaks. One wheel smelled a little warm, but no smoke or pull like it was a caliper sticking, matter of fact, that caliper is only about six months old as I have replaced it when I put new pads all around. After a hour or so in Home Depot, got in it, brakes felt normal, and have ever since.

Reckon what it was? Master cylinder bypassing internally? I’ve had bad masters before that would do that when they got old, but once they started they kept doing it, they didn’t go back to normal. Don’t see any line leaks, either, but I haven’t got under it and looked up close. No fluid on the tires.

Not really wanting to just throw parts on it to see what works, but I’m good with a master cylinder if that’s what it takes. Ideas?
It sure sounds like a master cylinder getting ready to fail. that one time event where the pedal went to the floor could have been a warning shot. I've seen many masters leak at the rear seal, between the master and the booster. Often you don't see that until you pull the master.

That said, a few weeks ago my wife called me and told me her brake pedal went to the floor. I took brake fluid with me, just in case. The master was empty! I got in and pumped the pedal a couple times and the brake pedal was rock solid. Couldn't see any fluid leaking. We were about 6 blocks from home, and she elected to drive it home. As she stopped in the driveway, she said the brake pedal went to the floor again. The master was empty again. One rear wheel had one little spot of fluid under it.

Blew the wheel cylinder completely out, the entire inside of the drum was full of fluid. The shoes looked great, except for being covered with fluid.
I replaced both wheel cylinders and the brake shoes. It took nearly 2 master cylinders of fluid before it showed anything under the car.

I would expect the master is going out on your truck if you are not loosing fluid. I would at least get under the truck and look the lines and hoses over real good. I might even pull the drums and take a look at the condition of the wheel cylinders. I have experienced a master that would only bypass under certain conditions, but after the 2nd time (with long a period of time between), it got replaced. I did buy a master and had it sitting on the bench. I changed it out because I finally figured it would probably totally fail at about the worst time to change it. There was about 6 months between that 1st time, and when I replaced it.
That was the puzzling part, it wasn’t low of fluid. I’ve had one leak into the booster before, I had forgotten about that. Hasn’t leaked much though, still full, so I’m guessing it bypassed internally. Think I will go ahead and order one and have it here, probably a good thing to replace anyway, this one has 175,000 miles on it if it’s the original one so it’s probably like me, tired.

Thanks Gene.
In the past 4 months, I've replaced three brake boosters for that exact reason.
Put pressure on the brake pedal, start the engine and the pedal went to the floor. Yours may be warning you.
I had a similar issue on one of my dailies. Sitting at a light, the pedal would keep dropping. It was the master.

Change the master. At this point it doesn't owe you anything.
Went ahead and bit the bullet and ordered a booster and master cylinder today. Since it kinda either or, I’m just going to do both at the same time. Not but four studs difference so not much difference time wise. Better safe than sorry later.

Now if these temps would moderate a little back into the 80’s it would be a lot easier. I’m ready for fall…:rolleyes:
Wise plan man...

What he said!

I've only totally lost brakes once & flew through a very busy intersection that scared the **** out of me. After that, I use issues like that to replace the master & never look back! I've also had problems with vacuum leaks & a leaky booster.


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