Pedal goes to the floor... And i am stumped

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ragtopfury

ragtopfury

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So I have been chasing a floored brake pedal issue in the Shelby Dakota. Pedal went to the floor in the driveway - lucky me. Barely enough brake to hold it from rolling at idle. No leaks, master is full. Assumed bad master. Bought a new one. Bench bled, then bled at all 4 wheels until nothing but new fluid came out. Good solid, bigh, hard pedal..... Until i start the truck. Then it goes to the floor with very little effort. Bled em out again. No better. The pedal is dang near to the floor when depressed, with little effort. Can hear and feel the brakes working at all 4 corners, but i have never had a pedal so low. Thought maybe the power booster was failing, so bought a new one. Same issue.

Thoughts?
 
If one of the hoses were failing, would i see it buldge/balloon out when the pedal is pushed?
 
yes. Power brakes. The pedal is so low, it's only about an inch off the floor at a normal stop. A hard stop puts the pedal on the floor.
 
Hi,

On the master cylinder, just because it's new doesn't mean it's good especially if it's a remanufactured unit. I doubt you would see a hose balloon out , they are pretty thick internally. How did you bleed the brakes? You still may have air in there.

Also, are the rear brakes adjusted properly. They need to be adjusted all the way up or you will get a low pedal.
 
Volume in caliper bores can be an issue also. Some of the newer assemblies have lining of half inch thick or more. Wear those down to nothing and the pedal must travel further. This displacement issue is very common for hydraulic clutch slaves.
Drum brake wheel cylinders make less noticeable difference of course. Worn linings there would add to the total displacement area when those self adjusters fail.
 
yes and no. While sitting in park and running, a vacuum power assist will let you push the pedal a lot farther than is required to stop. Anti lock systems do also. Best pedal travel test is with all systems off.
 
How level is the master cylinder mounted on the truck?
 
Thoughts?
Click to expand...
seperate the mcyl from the booster & check the rounded nub clearance to the bottom of the piston recess in the back of the mcyl. want ~.020". If good there then confirm the rear linings are spooned up & you can do that for testing purposes by pulling the ebrake. confirm front pistons are out/pads against the rotors. If all good there then there is still air in there. some caliper are a MF to get bled out. You might with a helper: pump the pedal fast for several strokes then have him open the bleeder on the down stroke (good timing needed) &make sure he closes it fast before you let up. extreme case you need to remove the caliper with hose still attached & tilt it then bleed some more. short version: (1) rounded nub clearance (2) air
 
The issue started before i replaced the master. I went the expense of a new unit as opposed to old. No other brake work was done. Got home from vacation, got in the truck to move it out of the way and the pexal was low. Next day it went to the floor. Pads are nearly full lining, and appear to be functioning. Rears are adjusted and functioning as well.

Master is in its original, level mounting position. As for vacuum, i have bypassed all emissions/vacuum junk and am running a carb, stand alone ignition, and the only vacuum lines connected are the dizzy advance, heater control and brake booster. No open ports remain. Bleeders are at the top of the calipers and i get no more air out of them. Initial bleed was with a vacuum bleeder, then old school method of a helper pump 5x, hold, i crack bleeder loose and close before letting pedal back up.

I will check the booster to master rod clearance this weekend, as well as do another bleed out.
 
Bleed from the wheels up if you can, because some of those damn things just won't bleed the air out by pushing the fluid down (air bubbles try to go up) :D

Powered bleeders are almost a must in these cases, but I have done it using a squeeze bottle. (PITA, but it worked)
 
The power brake booster is bad, or the check valve on the top is faulty.
I've had this happen to me a few times.........
 
Dartnut said:
The power brake booster is bad, or the check valve on the top is faulty.
I've had this happen to me a few times.........
Click to expand...

Replaced both, already. Will bave to maybe try another, as sometimes new parts are defective.
 
ragtopfury said:
yes. Power brakes. The pedal is so low, it's only about an inch off the floor at a normal stop. A hard stop puts the pedal on the floor.
Click to expand...

That's your first problem, ditch that booster. Problem will be fixed. No need for them on an a-body, they just make sponge cake. But this advice falls on deaf ears on most of the membership. Everybody wants a booster because everybody else has one. :roll:
 
I'm not by any means a brake master but have done a lot of brake jobs in my time. One time I did a master cylinder replacement on a Chrysler lebaron ,I think it was around a 98'
model. I bench bled the master and all four wheels and had and good pedal until starting then no pedal at all. Pulled the master bled it again and all 4 wheels with same results
and fooled with it for about 3 hours checking the lines,hoses and wheel cylinders and calipers. Finally had someone fire it up and pump the brakes while I bled and problem solved.
 
Ragtop, this may sound stupid but does the truck have abs brakes? If so I have seen this happen on a couple of other makes ie. ford . But the abs system had a dump valve that opens during antilock situations. It sounds like you've got everything else correct. I don't know these vehicles that well if it has an abs controller of some type, try unplugging it, then see what happens. Hope this might help!
 
PoisonDart74 said:
That's your first problem, ditch that booster. Problem will be fixed. No need for them on an a-body, they just make sponge cake. But this advice falls on deaf ears on most of the membership. Everybody wants a booster because everybody else has one. :roll:
Click to expand...

Funny thing is you don't need ears to read which is something you can't do apparently. He is talking about his truck not his A-Body.
 
dwhip361 said:
Ragtop, this may sound stupid but does the truck have abs brakes? If so I have seen this happen on a couple of other makes ie. ford . But the abs system had a dump valve that opens during antilock situations. It sounds like you've got everything else correct. I don't know these vehicles that well if it has an abs controller of some type, try unplugging it, then see what happens. Hope this might help!
Click to expand...

This is what I found in the circuit hence the reason I bled them while running
 
Pull your rear drums and check the wheel cyls, pull the boots back and make sure they aren't wet. I've seen wheel cyls weep and suck air back into the system causing low/no pedal.
 
I believe that truck has a RWAL brake system so you may want to look up the procedure for bleeding, might be more involved than just the old pump and hold especially if you drained a lot of the fluid out.

I was a Chrysler tech. Sometimes when we did brakes on an ABS system we had to use the scanner to bleed the brakes especially when replacing major components like a master cylinder. That computer is pretty primitive/archaic so I doubt it would have anything to do with it but it's worth a look just to rule it out.

Check to make sure you have everything plumbed correctly - proper lines going to the proper ports, etc. and you didn't switch the front and rear circuits somehow.

Also, disconnect the vacuum line from the booster and cap it off and see if it changes the pedal feel. That might help to nail it down. Power brakes without power assist will be tough to operate, it will take a lot of effort for very little braking power.
 
PoisonDart74 said:
Great attitude son, you'll go far! :glasses7:
Click to expand...

Coming from the guy that was just busting the OP's balls because you couldn't read. You sir should be a politic with the ability to talk out of both sides of your mouth like that. :burnout:

And I'm not your son...
 
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