spongy brakes? HELP!!

[GMC3]

just installed entire new brake system from M/C to pads. bled them multiple times and all of the air is out of the system, but my pedal is still spongy. if i pump the pedal 4 or 5 times it builds pressure and holds it, but the next time i hit the pedal it goes to the floor. double checked for leaks and none. I'm thinking maybe a bad M/C, but it is a remanufactured one from NAPA. Is it possibel the rmfg M/C is bad or am i missing something

[Robert1970C20rstbukt]

Make sure the MC is full of clean brake fluid. Also are these manual or power brakes? If they're power brakes the booster has a lot to do with how the pedal feels.

[GMC3]

it is power brakes, but the booster vaccuum isn;'t hooked up. I just figured that it would feel like manual brakes until I hooked up the booster. that's how it feels in my f150 before i start the engine and the booster kicks in

[cableguy0]

with power brakes unassisted the pedal should feel rock hard. its very possible you have a bad master cylinder out of the box but did you bench bleed it??

[GMC3]

no i didn't. how do you do that. Is it necessary
thanks

[cableguy0]

yea its neccesary the master cylinder shoulda came with some instructions. you can do it on the truck. just bleed it like you would anything else in the brake system have someone pump up the pedal and crack the line open till the pedal sinks to the floor then tighten it back up. have a rag wrapped around the line fitting area otherwise u will get fluid all over your paint

[Longhorn Man]

After pumping, does the pedal stay up for quite a while...or does it end up sinking.

If it stays up as long as you hold the pedal down, then you should look at shoe adjustment.

[greasemonkey]

Check for leaks at all the fittings too.

[special-K]

I`m having the same problem,all new stuff.I need to go over it.I had a flunky do it,he needed the $,now he`s in Fla.I need to go over it myself.He did bleed the m/c first.Also,no e-brake,could be adjustment.

[GMC3]

bought new m/c, bench bled, bled at all 4 corners and still having same exact problem. no leaks at any connections .

[cableguy0]

did you replace your hoses? its possible they are expanding and giving u the spongy pedal. other than that i would have to say there is air in the system somewhere still. when ya start bleeding it start furthest wheel from the master cylinder so passenger rear the driver rear then passenger front the driver front to end it all. rear brakes out of adjustment could also cause the spongy pedal. is this thing 4 wheel drum???? if so adjust all the shoes

[Gordo]

Sounds like brake adjustment to me too, assuming the drums(?) are within specs.

[ChebyCheyenne]

if the pedal feels stiffer after you stroke the pedal a couple times it means you have air in the system somewhere.

[4x4Poet]

Good info above. I'd like to reiterate some of it:

With the rear (and front, if so equipped) shoes set to far from the drums, the pedal must be pumped several times to pump enough fluid thru the lines to move the wheel cylinder pistons the extra distance necessary to make the shoes contact the drums. IOW, the one pump of the pedal only moves whatever the master cyl can pump in one stroke, but that single movement of fluid may not be enough to move the wheel cylinder pistons the excessive distance backed off shoes require to come in contact with the drums. Just the long way of saying what was said above.

I don't know the exact way to set the drums. I just set them so that the drums scrap the shoes as I push the drums on by hand. There's no fear of wearing the shoes out at start up since the adjusters will set the shoes even closer the first few foward or reverse braking motions. BTW, did you install new adjusters? Old ones will often stick and not adjust like they should, either self or via braking action mentioned.

The fluid bleeding sequence cableguy0 gave is spot on. So is the hose question. Bend the hoses around by hand to search for cracks that would let inner rubber hose balloon out thru a crack. Also, if the hoses are the original 67-72s, they need replacement. Period. No question.

Air in the system is another cause, well-addressed by cableguy0. I'll just add that old fluid has moisture in it and should always be flushed out during a brake rebuild. Make sure you have bled each wheel till fresh fluid comes out.

Bleeding the master is also necessary, since air in the master will cause your truck's systems, though usually, air in the master will cause the pedal to go almost to the floor with no build up after pumping the pedal.

Like I said, just playing off everyone's good advice.

[GMC3]

all of the hoses are brand new, the drums are adjusted and i don't see any leaks anywhere, Their must be air in my lines somewhere. maybe because i started with a system with no fluid in it, it's a @#$% to get all of the air out. i'm going to go bleed them a few more times.

[nevada70chev]

i did all 4 corners new, i'm going to take mine in to be "pressure bled" and double checked i did it all correctly.

[Longhorn Man]

GMC3, when you pump up the brake pedal, does it stay pumped up as long as your foot is on it...or does it sink down after a min.
This will tell you what is wrong. Pressure bleeding is not needed (on a stock system) onless you have synthetic brake fluid, which is crap anyways.

[GMC3]

kept bleeding and still getting nowhere. I am convinced there is no air in the system and i can see there is no leaks. Maybe sometrhing is wrong with my setup itself.
M/c '72- remanufactured -was going to use a alater model but the primary pistons are different and the later model does not fit on my booster.
Prop. valve '78- brand new from dealer- because my frame is a 69 I put a 78 front end on it for discs and got the prop valve to match it
all lines and hoses are brand new along with the calipers and wheel cylinders
Drums are adjusted also
PLEASE HELP!! thank you

[Gordo]

Double check that master to be sure it's for a disc brake system and not drum brake. Also with most master cylinders with a double bowl, the front bowl feeds the front brakes, the rear bowl feeds the rear brakes. There was a few in the early disc brake years that were opposite, front bowl fed the rear and rear fed the front. In most cases the larger of the two bowls feeds the front, but once again some of the early ones had two bowls the same size. Just some food for thought.

[72K10]

I redid all of my lines recently and had the same problem. It took two master cylinder bench bleeds and three or four complete system bleeds to get all the air out. If your system is spongy, it's either air or water in the lines. But you can bleed both out. Just bleed the cylinders until the brake fluid is clear.

Couple of important things other folks have brought up that could lead to your problem. First, you've got to bench bleed the master cylinder. It only takes 30 minutes or so to take it off, bleed it, and put it back on. You'll need to use the bleeder kit that came with the master cylinder or buy a self bleeder to do it.

Second thing is that you might want to hook up your booster. I could not really bleed mine until I got the engine going and revved it up a little to boost the vacuum. Once I did that, the system pressurized for better bleeding.

[4x4Poet]

GMC3, have you taken the primary advice given? i.e., bled the master cylinder with a master cyl bleed kit that should have come with the master or is avail at parts stores? The re-mfr'd master should have come with bench bleeding instructions, though, as Cableguy0 first mentioned, I believe you can bleed it on the truck as long as the master sits fairly level.