Panic Stop & Loss of Brakes

[icanfixall]

Attention brake experts. If I need to really stand on the brakes I get one tiny squeak from the rear tires.. Then not much in the way of stopping power. Even the front climbs back up some. I do end up stopping but its unnerving to say the least. I'm running the hydroboost from a super duty truck that had the 10 lugs on it. I have the large 1 5/16 mastercylinder and running it off the stock power steering pump but I got the resivour with the 2 retuns to it. I have kept up on the rear adjustments too. If my parking brake gets down about 1/4 of the way to the floor I adjust the rears. I'm able to stop easily if I just hit the brakes in a normal way. But if I hammer them its almost a free ride into what I'm trying to avoid. I don't let others drive this rig either. Any ideas out there...

 

[91idi]

Rabs working?

 

[gonecrazyi]

Blown rear axle seals possibly getting grease on the brakes? That's what mine did when that happened to me.

 

[Dsl_Dog_Treat]

Gary was that a remanned unit?
It's possible that one of the seals is failing inside the unit.

 

[icanfixall]

The rears work fine and nothing on the shoes like grease of gear oil. Been looking at that idea too. The hydroboost and the master cylinder was off a wrecker in the yards I shop at. I'm not one that buys wrecking yard brakes or master cylinders but.... I found this complete unit and it was just installed before the truck was junked. It had a flat bead on it so it was a work truck and a ZF was in it but that was gone. I really can't say if this was a new or rebuilt unit but the master cylinder resivour didn't have any greasey finfer prints on it and the booster was just as clean. I"m thinking about bypassing the rabs but its about 8 years old. Maybe less. I do recall replaceing it myself but thats been a while ago. Even made up my own custom pressure bleeder and it worked great... A one man bleeding system is what I am... I never get nor do I ask for help... Everyone around me is flat ass useless.... A bunch of short bus window lickers....

 

[91idi]

By pass the Rabs and see if it works. I bet you triggered it.

 

[SHARPIE]

Sounds like you still have a little air in the system...Gets hot and expands try re bleeding them, Any time I have someone help me bleed brakes they want to pump the s**t out of them in 2 seconds then hold...All it does is create foam which is harder to get out...So I do them myself too.

Rear drum brakes need a small amount of drag just a tiny rub,Also your drum itself might be worn beyond its limits.

Take it in gravel and slam on the binders and look at which ones are working if its front or rear.

 

[Jake_IN]

By pass the Rabs and see if it works. I bet you triggered it.

I'm with scooby doo on this one! I bet that Rabs is the culprit.

You must be registered for see images

 

[towcat]

But if I hammer them its almost a free ride into what I'm trying to avoid. I don't let others drive this rig either. Any ideas out there...

gary-
before you panic, got for a test drive and find yourself a large empty parking lot. get the truck up to speed and hammer the brake pedal. if you don't have any braking power after a short delay, it's time to replace the hydro unit.

also-
click on the linky for some reading.
http://www.scribd.com/doc/6829161/Operation-Diagnosis-and-Repair-of-HYDROBOOST

 

[hesutton]

... Everyone around me is flat ass useless.... A bunch of short bus window lickers....

Gary, I don't have anything new to add, but I really thought that quote was great. Laughed for a while at that one.






Heath

 

[gonecrazyi]

I've been working on an 01 e450 RV that's doing the same thing. I has new rear calipers and a new master for the brakes. I've bled and have it to where the fronts will grab but the pedal goes almost to the floor and the backs dont grab. Its had me puzzled for awhile. I never really attributes these symptoms to a bad booster though.

 

[ironhorse2021]

I've worked on many RABS valves in the lab (20 yrs ago). A quick test you can perform with minimal tear up to your vehicle is the following:

1. On the end of the RABS valve opposite the wire pig tail is a large hex endplug. Remove it and you'll see a spring and below that a piston.
2. Check the interior bore of the this cavity to make sure it's in good enough shape for an oring to seal on - no rust or pitting.
3. Remove the spring and reinstall the endplug.
4. No need to bleed anything
5. Do a strong brake apply - this will move the piston you were just looking at to the back end of the bore. Without the return spring, it will stay there.
6. Now, go do a few normal or high effort brake stop and see if things feel better ?

If this fixed the problem, you have what is called a dump leak into the fluid accumulator. When you apply the brakes, fluid goes into that piston chamber instead of out to the brakes. Then, when you release the pedal, the spring pushes the fluid back to the MC reservoir. Typically, this failure mode will also have a long pedal during braking (close to the floor).

The RABS unit really does add safety to these vehicles by keeping the rear of the vehicle behind you on a hard stop. Huge rear brakes and now weight are a bad combo. RABS doesn't do much to improve stopping distance - just stability.

If you have a bads RABS valve, I recommend you replace it. If the test above doesn't change the pedal feel, give me a PM and we can try a few other tricks.


T

 

[LARRY RANALLO]

... Everyone around me is flat ass useless.... A bunch of short bus window lickers.... that is funny. i had too pm you, thats the first time for me on OB towcat and ironhorse had some good inf.

 

[icanfixall]

Thanks everyone for all the information and ***. I will be diving into this issue and see what I find. As always, I'll post what I find..

 

[riotwarrior]

... Everyone around me is flat ass useless.... A bunch of short bus window lickers....

***** ..Gary that made my day..and my sig...