About 1 out of every 50 stops, the pedal drops and feels like its lost power assist. No Brake or ABS lights come on in the dash, and no codes on odometer. Not sure but kinda feels like steering loses a little power as well. As soon as you get it stopped, its normal again for awhile. Normal braking at speed, and slowing down, just fails from 5 mph to zero when coming to complete stop. Brakes do feel a little softer than usual, but that might just be from driving wife's car for awhile.
Brought it in for service, and luckily it happened to the tech. He said the ABS light did come on. They couldn't find anything wrong, so they threw the $450 ABS module on. Brakes did the same thing to me again the next day.
Search results here suggest RW dump valve, but also say you can't find them. Does this sound like dump valve failure, or something else? Just moved, and haven't found a good tech like I had in NY. Can't afford to replace everything.
Thank you for any help.

 

Here's a cheap thing to try..... The power steering and brakes share the same pump and fluid. This fluid gets really hot too and probably is rarely ever changed by anyone EVER. Who replaces their PS fluid????? Changing this fluid is a great idea and changing it with some quality synthetic is a better idea.

Go pick up a quart of quality synthetic power steering fluid and a cheap few dollar plastic hand pump with a few feet of clear hose. Stick the clear hose in the power steering pump and suck out all the fluid you can. You'll probably only get around a pint. Then add a pint of the new synthetic and drive it around the block making more turns than normal. Then repeat the process and add the remaining pint of synthetic PS fluid. It may just be what your PS pump is looking for.

 

Thanks, will do. PS fluid looks clean and full. Brake fluid looks full, but dark.
Been reading about disconnecting ABS system to fugure out if it's sensors or mechanical problem. There is an ABS fuse under the hood, and a RWAL fuse on side of dash. Should I pull both and test it out?

 

I couldnt say one way or the other if pulling those fuses will help diagnose anything but with that said I'd only tend to not suspect the ABS if the module was just replaced.

There's coincidentally no drop in braking power when you're steering or visa versa?

 

Changed PS fluid out with royal purple synthetic. Brakes did not fail during test drive, but may not have tested long enough. Still feel soft. Will try to bleed today, and double check for leaks. Or maybe find a shop to do complete brake fluid change.
Every time they fail has been in a parking lot or base gate traffic. Seems like if I use light pressure too long, pressure eventually gives out. Hopefully not master cylinder?

 

Matt, a master cylinder would be a good news story. They are easy to diagnose and pretty easy to replace yourself in the driveway. Oh yeah, they aren't even all that expensive.
If you sit in the truck with the engine running and apply firm pressure to the brake pedal, does it slowly drop to the floor?
If so, you master cylinder is done.

 

I agree with Tallguy67. I only leaned towards pump/oil lubrication issues because of the noted random loss of braking. Maybe the fluid change did the trick.

Also you can have the brakes bled if you really think its needed, but I've never been a believer that brake fluid will get bad enough to keep the brakes from working. Yes brake fluid is hygroscopic which will cause discoloration and corrosion over many many years, but a sealed system will remain sealed and functional until corrosion or other component failure allows contaminants like air to enter. So I'm not saying bleeding the brakes is a "bad" idea but it wouldn't be high on the list unless the brakes were constantly mushy and you had to pump the pedal for any positive action. Otherwise you're merely opening up another can of worms and creating another variable in this puzzle you're trying to solve.

 

Thanks. Just tested again. Brakes hold firm at a stop with no pedal drop. Bleeders are pretty corroded, so I will leave that alone as you suggested. Couldn't get a failure again today, maybe the PS fluid was that bad. I may hit you back up if they fail again.
Thanks again.

 

Either it's getting worse, or I was being lazy. You are correct, with engine running, I can get the pedal to slowly drop to the floor. It holds pressure when you're light on it, but drops away when you press harder.
I will start shopping for a master cylinder, and reading how-to posts.
I appreciate the help.

 

Glad you solved it Matt. The last time I did a master cylinder, it came with instructions. I followed the instructions and did just fine. On that particular vehicle, bench priming the master cylinder was really important and easy. I don't remember what I did but I remember that I just followed the instructions that came with it and I did fine.

 

Hope I don't wear out my welcome, but wanted to make sure I understand. Engine running, in neutral, parked on a hill, the brakes will hold. If I hold that same pressure, pedal feels firm enough. If I add pressure, the pedal does go down slowly, like twenty seconds to almost bottom out, but still holds the truck. I did not force pedal all the way in case that hurts something worse, but brakes never released. Brake Pedal makes a whoosh or hiss noise when you let the pedal back up.
Any idea what size or where to buy plugs for master cylinder to test for internal leak, or you pretty confident I should just change it out?
I appreciate the help, just wanted to make sure I described the problem accurately.
Thanks again.

 

Interesting, I think I have a similar problem.

Every once in a while, when I hit the brakes it feels like I'm only getting half the braking power. Sometimes it will happen when I first hit the pedal, other times it will start strong then suddenly drop off. It really feels like the rear brakes releasing, although I can't confirm it.

As for steering, occasionally I can feel a little drag or "buzzing" feeling through the wheel when turning just off center. The power steering fluid is fresh. It has a new steering box and I had to replace a leaking hose to the brake booster so I emptied and refilled the pump. The other new development is that at full lock, it will sometimes not want to return if I'm driving forward. I have to back up slightly and straighten the wheel, then move forward again. Not sure if that's related or a whole other issue.

 

Try this..... With the engine off, press on the pedal. It should be firm but under normal conditions you can pump it down. With the pedal down to the floor (or close to), hold the pedal and then start the engine. I remember reading that a properly function hydro-boost system will cause the pedal to push back up into its normal position even with your foot against it.

 

Matt, in my opinion you have perfect symptoms for a failing master cylinder. Keep in mind I am not a licensed mechanic but the way I was taught to diagnose the master cylinder is to see if I can get the pedal to slowly fall to the floor. Brake fluid won't / can't compress (otherwise it would be rather pointless and useless) so in a properly bled brake system that is working well, you should be able to push the brake pedal down until the brakes are all engaged and then go no further. If the seals around the pistons in your master cylinder are worn, then fluid will leak past the pistons and allow the pedal to keep dropping under steady and firm pressure.

I hope that helps.

 

I agree with this completely EXCEPT the OP explained that what he's experiencing is that the pedal will drop and the brake power didnt change. Also describing that it "seems" like there's less braking power but potentially thats only because he's loosing the hydro-assist which forces him to have to push harder to achieve the same braking power. I'm second guessing the master cylinder as the culprit only because if the MC was bad and the pedal was allowed to drop then that would also mean air would enter and the whole system would eventually become soft, as you explained, brake fluid does not compress. But in saying this, if the pedal does drop and the brakes still work then potentially this is a bogus hydro-assist pump causing all the problems.