So i have replaced the MC, all the brakes and have bled them very well(including bench bled) I know there is no air in the lines or the MC. Problem is the pedal goes 3/4 way to the floor. Truck stops fine but the pedal is super soft for my liking. I did add one ton wheel cyl's awhile ago but i dont think thats it since they arn't leaking and they don't take the much more fluid to fill. I also used the right MC for my truck (its the 8800lb one). Any ideas guys? I have seen a lot of people with this issue and no real fix for it. I can pump the brakes when its running and they won't get harder. and the pedal is firm with the engine off after pumping a few times.. I am at a loss now...


Josh

 

did you manually adjust the rear brake shoes up tight (slight scrub). ?
i like to adjust to where its locked up, then back off until it spins with just the slightest of ticking/scrub.

the proportioning system won't allow the front brakes to really work hard until the rears kick in.

 

Yup as i said rears are adjusted

 

If the pedal is spongy, there is still some air in there...... Draft a friend, have him pump the brake pedal three times, hold, and then you crack open the bleeders. Go all the way around the truck, starting at the passenger rear, then drivers rear, passenger front, drivers front. Each time, build some pressure, then release the valve. Do that several times at each wheel. See if you get any air. (I am guessing you will.)

If the pedal has a lot of travel before anything starts to happen, rears need a bit more adjusting. When you adjusted them up initially, did you step on the brakes a couple times before/during adjustment?

Did you bench bleed the M/C before you installed it?

 

I have bled the brakes several times so i know there is not air in there(and yea i bench bled till i had no bubbles) Im beginning to wonder if my old MC was bad and this new one i got is bad also...

 

So is it spongy, or low pedal? Or both.....

 

not spongy.... Just goes down to far(almost to the floor) and if i pump the pedal it wont firm up at all.

 

Are you sure that both circuits are pressurizing properly? The symptom sounds a lot like a failed master cylinder that's pressurizing only one circuit.

 

would a strong squirt of brake fluid from both front and rear when bleeding prove that both circuits are pressurizing ?

 

The pressure in a sealed system is a helluva lot greater than it gets with a bleeder screw open, and a master cylinder piston seal that won't work to make useful line pressure can often make a fine squirt out of a bleeder screw.

Have you observed the soft lines for swelling under pressure? The brake system doesn't really move a lot of fluid in one piston stroke (as you can see by the narrow cylinder and short stroke of the piston) so any swelling will rob you of system pressure. Run the engine for a moment to get the booster charged, then with the engine off and while you observe a soft line, have your assistant stand hard on the pedal with both feet, pushing like he wants to break off the driver's seatback, and hold that for 30 seconds. There should be no visible swelling, and the pedal should not sink after full force is applied. Repeat (starting at charging the booster) for all of the soft lines. Any swelling or pedal sinking is indicative of a problem.

The not so wonderful part: It often happens on older vehicles that boosting the system pressure with replaced components exposes the other failures that were just waiting to happen.