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

[AJ/FormS]

You have a rock-hard pedal with the engine off.I have never seen it where a rock-hard pedal had hydraulic problems. A Rock-hard pedal means your foot is directly linked to the wheels.
And you say fresh fluid is coming from every port.That means at near zero pressure, there are no restrictions in the lines and hoses.
And you say the rear brakes are properly adjusted.
And the front pads are nearly new.
And you can hear the brake parts working at the wheels.
And there are no external leaks.
And the pedal drops with a toe on it, when the vehicle starts up.
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But you say the problem does not show,until the vehicle is started, and it did it before the booster was changed.
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Let's assume that you know what you are doing, and that everything you have said is true.

Did this truck come home with you from vacation? Does it have ABS? Does it have a rear, bed-mounted proportioning valve?
If no,no, and no; then I believe that there are just two possible issues left. The booster or a demon.
The least likely, is the booster,cuz it behaves correctly, perhaps too much so. That leads to a pushrod adjustment.
But the most likely, is that something mechanical is fooling your hydraulics.Somewhere, something is acting as an accumulator. I believe it will be in the rear system. When I come to this point, I like to isolate the front from the rear, and work on just one end atta time.But before that, I think it would be a good idea to check the park-brake system. If it were to be on or partly on, during the rear shoe adjustment, it could easily mislead you into thinking everything was A-OK back there.

 

[ragtopfury]

Easy fellas.... and thanks to all who have opined here. I have plenty to try here, some of which i have done already and in the past on other vehicles...

No it does not have a full abs system. It is an 89 Dakota and does have the RWAL valve near the rear as rmchrgr mentioned.. I put an MC on this truck about 10 yrs ago and didnt have this issue bleeding them out.

I will report back after the weekend, assuming life allows me time to dive into it by then. Just found out the wife is headed out with the middle and high school girls from church for a weekend retreat. Trying to get this done with a 7 yr old and a 5 yr old might be a bigger challenge...lol

 

[ragtopfury]

You have a rock-hard pedal with the engine off.I have never seen it where a rock-hard pedal had hydraulic problems. A Rock-hard pedal means your foot is directly linked to the wheels.
And you say fresh fluid is coming from every port.That means at near zero pressure, there are no restrictions in the lines and hoses.
And you say the rear brakes are properly adjusted.
And the front pads are nearly new.
And you can hear the brake parts working at the wheels.
And there are no external leaks.
And the pedal drops with a toe on it, when the vehicle starts up.
------------------------
But you say the problem does not show,until the vehicle is started, and it did it before the booster was changed.
-----------------------------

Exactly. Nice summary.

 

[Inertia]

You say you can "hear" the brakes function,, if so,, it can only be the rear shoes moving on the backing plate,, I know that sound,, if you can hear it,, it's moving too much,, again, adjust..

that's what???.. half a dozen well known, reliable folks suggesting it...

cheers,,

 

[ragtopfury]

No, the truck didnt come home from vacation with me, it has lived with me since March of 89...lol.

Rear brake adjust - very slight drag when turning the wheel by hand. Been that way forever every time i adjust them... Besides, the brakes worked just as they should prior to vacation. Got home 16 days later and they go to the floor. Again - no leaks, for anyone who missed it.

Ajforms has the summary dead on.

Will report back after the weekend when i get a chance to try the many things mentioned in the last page and a half .

 

[AJ/FormS]

What

 

[AJ/FormS]

"No it does not have a full abs system." As oppossed to?
"It is an 89 Dakota and does have the RWAL valve near the rear as rmchrgr mentioned."
So that would be; no-no-yes.
Well then just be sure the link rod is secure on that valve,in addition to the rear checks.
The absolute fastest way to isolate the front from the rear, is to clamp the rear flex hose; but be prepared to replace that old hose, cuz they don't always survive the ordeal.
If your starwheels are working, you can crank the shoes out until they are thoroughly jammed against the drums, and retest the booster. Just be sure the park-brake is FULLY off. This supersedes the clamp-method.
If the problem persists; make sure your front wheel bearings are properly adjusted, and that the calipers are free to slide on their ways.And retest.

 

[nm9stheham]

Trying to get this done with a 7 yr old and a 5 yr old might be a bigger challenge...lol

I'll bet your 7yo can be trained really quick to pump the pedal properly while you bleed at the wheels!

 

[ragtopfury]

Took the day off work. Got it figured out.
Never messed with the rwal valve before, but see a bleeder on it. Bled from there - no change....

Rapid robert nailed it early on in this thread.... Pulled the new master off the new booster to check rod length. Using various methods of measuring, I have almost a 1/4 inch from the end of the rod, to where it meets the piston in the master. New master measures the same as the old one (which apparently is still good). Just happen to have a booster from an 85? Diplomat. Rod is 1/4" longer out the front, but the pedal rod is too short. Went and picked up another new booster and all is good!!!

Glad this one is done beating me up! Now i got to get the garage cleaned up and reorganized so i can hopefully start on the 68........one month next year........

Thanks to all!