Brake problems

[Randy Bush]

I have a 93 dually 7.3 factory turbo. It has the sinking pedal problem. Have put new master cyl on, new booster , new vacuum pump , changed the RASB unit from a good truck . bleed the living daylight out of it. No leaks of any kind. Any thoughts? About to start at master cyl block off then if good move down the line. Very frustrating to say the lease.

 

[Kizer]

I feel your pain brother. The only OBS that I own ('93 DRW IDIT) has the worst brakes of my remuda ... can't find a vacuum leak, nothing seems to be plugged-up ... not sure where to go either.
Good luck.

 

[Randy Bush]

I feel your pain brother. The only OBS that I own ('93 DRW IDIT) has the worst brakes of my remuda ... can't find a vacuum leak, nothing seems to be plugged-up ... not sure where to go either.
Good luck.

And yet with the engine off they are solid. Have though about using a master cyl off of a 450, kind of spendy just to try. I am going to try and get it in the shop and tear into it more. I will let you know if come up with something that works.

 

[Slicknik]

@Randy Bush

Hmmm interesting, have you replaced all the rubber hoses that are suppose to be holding vacuum?

Have you hooked up a vac gauge to your system and seen what happens to the needle when the truck is on and you depress the brake ?

How much vacuum does your system hold when idling ?

 

[dgr]

I would del the rabs. I don't see how anything vacuum related could cause the sinking pedal described.

Sinking pedal is either air in the lines or an external or internal leak. ABS modules and master cylinders can leak internally

 

[I Bleed Blue]

If it’s been bleed properly it’s most likely a flexible hose that’s expanding under pressure. Need to look at all the hoses with the peddle pressed down.

 

[The_Josh_Bear]

I would del the rabs. I don't see how anything vacuum related could cause the sinking pedal described.

Sinking pedal is either air in the lines or an external or internal leak. ABS modules and master cylinders can leak internally

If it’s been bleed properly it’s most likely a flexible hose that’s expanding under pressure. Need to look at all the hoses with the peddle pressed down.

I'm no brake guru but between the above two posts they've got all the bases covered, EXCEPT:
When you put the new master/booster in were you sure to measure the rod on the master according to the directions? If it's too long you'll always have that sinking brake pedal and always end up bleeding the heck out of everything and ending up blaming the master after days of frustration. (Ask me how I know! )

 

[Cubey]

I'm no brake guru but between the above two posts they've got all the bases covered, EXCEPT:
When you put the new master/booster in were you sure to measure the rod on the master according to the directions? If it's too long you'll always have that sinking brake pedal and always end up bleeding the heck out of everything and ending up blaming the master after days of frustration. (Ask me how I know! )

Out too far and you will also have very poor braking too, your brake pads will literally smoke from overheating. I did that to my F250 by mistake. I didn't make the same mistake on the RV.

 

[Slicknik]

@dgr , correct I agree it’s a leak (hydraulic or vacuum )

just curious to see how his system is running

Because if he has solid brakes when the truck is off but falls when the truck is on that would lead towards a vacuum leak don’t you think?



Doesn’t hurt or cost anything it’s a free test

 

[saburai]

Good luck!
Watching...

 

[saburai]

@dgr , correct I agree it’s a leak (hydraulic or vacuum )

just curious to see how his system is running

Because if he has solid brakes when the truck is off but falls when the truck is on that would lead towards a vacuum leak don’t you think?



Doesn’t hurt or cost anything it’s a free test

Can anyone confirm that this is correct? If so, sinking pedal engine off equald hydraulic leak?

 

[dgr]

@dgr , correct I agree it’s a leak (hydraulic or vacuum )

just curious to see how his system is running

Because if he has solid brakes when the truck is off but falls when the truck is on that would lead towards a vacuum leak don’t you think?



Doesn’t hurt or cost anything it’s a free test

Nik,
I am guessing the extra force provided by the booster is allowing the bypass to happen when the truck is on. When it's off, the force he is providing with his leg isn't enough to make it happen.

You're right. Checking the vacuum is an easy check.

Josh brings up a very good point. Easy to test with two people. Open the master cylinder. Have one person watch the fluid and one press the pedal. The first 1/4" or so of pedal travel, you should see the fluid be pushed into the chambers from the ports in the bottom.

I would perform that test with the truck running and off.

 

[laserjock]

Sinking pedal is sometimes caused by rear shoes out of adjustment. Don’t know if that helps here or not.

 

[Slicknik]

@dgr

Hmmmmmm ok a good manual hydraulic test is in order then

While the truck is off , press repeatedly till all the vacuum is out and you have just a hard/firm pedal and you can’t hear anymore hissing from the booster

STAND on the brake pedal with 134 Nm of force which is about 99 ft/lbs for NO MORE than 3 seconds at a time (repeat 2 or 3 times..... let brake pedal rise fully after each attempt ) . If the pedal falls you have a leak and your hydraulic system is failing if the pedal doesn’t fall then assume it is sound and move onto vacuum testing your system.


Also I think the rod shouldn’t be adjusted outside factory spec , this is when catastrophic damage happens to the MC

 

[Randy Bush]

Well the vacuum running is 21-22 and pushing on pedal goes to 18-19 .so not vacuum which I was pretty of. Actually in my thinking if bad /low/ leak, vacuum it would act like when the truck is not running hard pedal ,but poor stopping power. There are no fluid leaks unless internal. So will dig deeper when get back of a short vacation next week. So it continues.