Spongy Brakes

[killer_mike]

Hey guys, i bled my brakes, got all the air out [as far as i can tell] but when the booster kicks in the brakes turn out spongy; i have a disk/drum set up, the booster i think is a 8-9 inch GM, the hoses are new, the entire brake lines are new along with the fittings, all its tight, no leaks.
I have a K10 67, long bed. But this is not any help because my truck dont come with disk on the front. Everything is pretty much custom.

But i dunno why the brakes are spongy, im thinking the booster might be messed up, or theres a possibility theres air in the lines.

Thanks.

[WorkinLonghorn]

Air in lines or in Master Cylinder. Did you properlt bleed the MC? Also,if the rear drums are out of adjustment,then there will be some travel of the peddle before the shoes are tight against the drums.You will need to try backing the truck and applying the brakes a few times if the auto adjusters are working. If not you will need to turn the adjustment wheels manually.

[killer_mike]

Didnt bench bled the MC, i figure if theres air on it it will come out from the lines and about the rear brakes, i redid the brakes, put lube and new shoes and deglazed all surfaces.
The valve that goes on the booster for the vacuum looks loose, can move it with my fingers varely applying force; all equipement, lines and fittings are new except the booster, rotors and drum assy. MC i think it gives around 750-780 psi, is a 1" diameter MC so it should give anough pressure to stop a 1/2 ton truck, the calipers are the sames that should go on a 73-75 burban [had an axle swap]

Originally Posted by WorkinLonghorn

Air in lines or in Master Cylinder. Did you properlt bleed the MC? Also,if the rear drums are out of adjustment,then there will be some travel of the peddle before the shoes are tight against the drums.You will need to try backing the truck and applying the brakes a few times if the auto adjusters are working. If not you will need to turn the adjustment wheels manually.

[Cyclone44]

I would say it's air in the lines...JMO

[killer_mike]

Dang, so the vacuum lines can cause this?, i always say every day you learn something new. I forgot to post this too, the brakes are only spongy when the engine is on.

Originally Posted by Cyclone44

I would say it's air in the lines...JMO

[68gmsee]

You cannot compress hydraulic fluid. That's why everyone is saying that there's still air in the lines somewhere...

Now, if it's really not air in the lines, then it is possible that the master cylinder itself is defective. Fluid could be forced past the seals in the piston/pistons and that makes the brakes feel spongy.

I'd bleed the hell out of them again and see if there is absolutely no more spitting or bubbles coming out. You'd be surprised how hard it is to get all of the air out sometimes. Be sure and use proper bleeding procecures with the hoses and containers to hold the excess fluid... Then if all is well and brakes still spongy, suspect the master cylinder.

[68gmsee]

Just reread where you said they feel spongy only when engine running. If that's the case, then it's possible the vacuum booster is bad. Seals may be allowing the pedal to feel spongy.

Here's a pic of what it looks like on the inside and how it works...

http://auto.howstuffworks.com/power-brake2.htm

[neonlarry]

If you did not bench bleed the master my bet would be that you have air in the system. How did you bleed the system? pressure bleed? pumping the brakes? I have had brakes where I have pressure bleed them with 2 qts of fluid and still had air in the system, not much but still had air.

[killer_mike]

I just checked the vacuum lines, reflared a piece of aluminum that holds the hose, its a weird carb, i say is a Rockwell [didnt do the engine swap] and tightened everything i could and still spongy.

I bled the brakes with an assistant pushing the pedal, the old way. I will bleed the brakes again till i die from it, otherwise i will change the booster and/ or suck it up till i change the engine so i can put new hoses and all that, thanks for the help guys.

[neonlarry]

If you can get or make a pressure bleeder setup (I made mine out of a garden sprayer and pressure gauge) it will work a lot better. You cannot push enough fluid to expel air in vertical lines by pumping the brake pedal.

[killer_mike]

I got personal with the truck, bled the system for about 1 and a half hour and got a crap load of air out, specially from the rear, i had leak on there a few days ago but i fixed it, it was just a lose fitting.

They work way better now, a tiny bit spongy, but they dont go all the way down anymore and make the tires lock up when i slam on them. With some more miles the pads and shoes will cure and i be good, but i thing the little spogyness left would be the booster and lines, couldnt do much on that.

Thanks again, and neonlarry lets talk about that pressure bleeder system of yours, any pics.

[neonlarry]

Take a look at these from Jegs:
http://www.jegs.com/p/Motive%20Produ...FQ-kiQodeXFzWg
You can go to OSH etc. and get a small sprayer for a fraction of the cost, a small pressure gauge, a master cyl cap and a barb fitting and your set. For our trucks though you will need the early american universal adaptor at the top of the page.
After assembly just fill the thing up with brake fluid, pump the pressure up around 15lbs and then bleed away. You get a nice continous flow that will get the air out of the hard to bleed spots.
P.S. Always, always bench bleed the master cylinder.

[killer_mike]

Much appreciated, but i have another question. My proportioning valve is CPP, not the one with the bias, it has a fitting with a pin covered by a cap, is this a bleeder for the p. valve? That way, in theory, i could bleed the mc on the vehicle?

[neonlarry]

I would have to see it to be sure but it sounds like it works them same as the stock valve that pin should be pressed in ( with a clamp) when the brakes are being bleed.

[killer_mike]