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Argghhh...stupid brakes

Zeus33rd

Zeus33rd

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So maybe one of you guys can help me figure this out. I recently re-worked the rear brakes on my truck. It was a total mess in there...everything was rusted together. I don't think the rear brakes were ever working at all. :haha: New wheel cylinders, springs, hardware, adjuster etc. I had great pedal for a week or so, but then it starts sinking to the floor at a stoplight, to the point that the light comes on. I've gravity bled one of the large sized (quart?) bottles of fluid through the entire system. There is no air in there.

Is it the hydroboost unit? Maybe a bad oring/seal bleeding back to itself? Every now and then, the thing makes a noise just like a whining power steering pump...is this an indication of a failing part?

I did a bit of searching, but most of the stuff I found had to do with the vacuum booster trucks. I don't know enough about these hydroboost systems to know if there are any parallels? I have to shift to neutral at a stop light to keep from idling into the car in front of me. :eek1::haha:

I guess I need to renew my membership here too....lol
 
If it is sinking to the floor it is not likely to be caused by the hydroboost unless it is really stiff getting there. If the light is coming on, and it is sinking to the floor I would be checking brake lines and wheel cylinders. I know you said they were new but that doesn't rule out a defective part.

The other possibility might be the master cylinder itself going out and not pushing the pressure down to the brakes themselves.

On my hydroboost systems, if there is air in the hydroboost the pedal will be really hard to press because it doesn't have the pressure to boost it. When the pedal goes down like that it is typically caused by a failure in the system itself from master cylinder down.
 
The pedal feel starts out and is perfect while just driving around. It's only when I have to hold my foot on the pedal at a stop light that it sinks to the floor. Even then, it starts out perfect, then slowly sinks almost to the floor. I have a fairly low stall converter, so it takes some pressure on the pedal to hold it.

If the problem were brake lines and/or wheel cylinders, wouldn't the fluid level in the master drop? Fluid level stays the same all the time, no leaks anywhere.

Is there a way to test/verify a bad master cylinder? I'd rather not just throw money at it.
 
Not sure on testing, but everytime I have seen the sinking pedal it has always been the master cylinder is bleeding back through bad seals. It is basically letting the pressure back off of the brakes. I am sure someone else will chime in with a way to test it and ensure it isn't just air in the master.
 
After I typed that last post I decided to go out and look at the fluid level. Level was down lightly in the smaller reservoir and over filled to the point of dumping out all over the place in the larger one. (I just did the gravity bleed and fluid check thing this morning, so I know it was good then) Now that I think back on it, this has been the case every time I've pulled the cap off recently. Is it possible for fluid to leak from one reservoir to the other due to a bad seal/o-ring in the master?
 
Yes it is, and you just found your problem. The smaller resevoir is for the rear brakes, and it is most likely pushing the fluid into the larger resevoir because the seal is bad between the two.
 
Funny how I noticed that every time I popped off that cap and thought to myself, "Self, that's pretty weird. I wonder why it's like that..." Then I would get totally distracted by something else and the significance of that totally flew right by me...lol! Thanks for the help. :)
 
It is easy to get distracted on something like that. Just a little thing that I don't think I would have thought much about either. I would have thought it was weird, but that's probably the extent of my thoughts until hitting an issue like you are seeing now.
 
Its not unusual for one to be lower than the other, because disk brake fluid levels go down as the pads wear.
And if you top it off, then it overflows when you put on the new pads. Of course I would never do that........:whistle:

But, the other side should stay the same. If its getting more full, then that is your problem.
I'm sure others have seen it, but I never have.

Every time I had a MC go bad, it did the same thing.
Start out good and then leak down while I held it. But I never noticed it moving from on to the other. It always just leaked back into the same chamber.
Either way, that is a standard test for a bad MC.
Just hold pressure on the pedal for a couple of minutes, and if it goes down, its bad.

And, just for information, it was probably caused by your brake work. Not much you could have done about it, but I see it all the time when you bleed some brakes that had not been worked on in a while.

The bore of the MC has gathered crud and rust past where the pistons normally stop. When you bleed them, and press the pedal all the way down, the pistons go where no piston has gone before and it tears the seals.

Due to my growing up with my old Jeep and its tendency to lose brakes at least a couple times per year, I am picky about my brakes.
If I have to replace something and bleed the brakes afterwards, I just go ahead and replace the MC if its been on there for a while.

Cheaper than body work.............:eek1:
 
Yeah you gave us enough symptoms of a bad MC, actually you can tell before youchange it, open the cover and level both sides out by syphoning from the higher one to the lower one, then go in the truck and press the pedal and hold a couple of time till it goes all the way down, and then go check the level again, I bet you it will do that even faster without the cap holding some pressure.
I had one like that and thought it wa snormal, but couldn't figure out why my pedal was going down in traffic, and finally gave up and changed the MC and voila, it was fixed.:waytogo:
 
KidJethro said:
The pedal feel starts out and is perfect while just driving around. It's only when I have to hold my foot on the pedal at a stop light that it sinks to the floor. Even then, it starts out perfect, then slowly sinks almost to the floor..
Click to expand...

I had this very same exact problem. it was my Master cylinder.. I ended up rolling into a minivan.. at a red light..
and for me, my brakes seemed a little more Stiff than usual.
 
lectric80 said:
Yes it is, and you just found your problem. The smaller resevoir is for the rear brakes, and it is most likely pushing the fluid into the larger resevoir because the seal is bad between the two.
Click to expand...
I was way too tired last night when I wrote this, the smaller resevoir is the front brakes, and the larger is the rear. Wheel cylinders require more fluid volume to move the pistons than disc brake calipers.
 
Confirmed bad master cylinder. Swapped it out with a new one (not re-manufactured) today and all is well. :pimp1:
 
My crew cab has this very same issue, it's been parked for a while though. Thanks for the reminder to get after that issue.
 
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