Weak brakes. Spongy brake pedal.

[cwin74]

My Bronco has disk brakes front and back and I feel my stopping power is terrible. I stop, but nowhere near where it should be. Pumping the brakes does not stiffen the brake pedal much. I have to either initiate my stop well before I need to stop or mash the heck out of the pedal. I need some ideas what to look at.

New Master Cylinder. (79 corvette)
New brake front hoses.
Braided rear brake hoses.
Brake pads look good, adjusted to good height.
Brake rotors look good.
No leaks anywhere.
No squeals, squeaks or grinding noises when mashing on the brake pedal.

The mechanic claims he bled the heck out of the brake system and everything looks good under the Bronco. Could it be the proportioning valve? (zillacon's idea) I don't see any leaks around it, could it be bad? I'll check vacuum when I get home today. Could be the vacuum booster?

Thanks in advanced!

[Jesus_man]

Yes, a vacuum booster going bad will cause that spongy pedal.

I'd like to know Zilla's reason behind the proportioning valve. I don't understand how that could cause a soft pedal, but I don't know how they function, just know what they are supposed to do.

J.D.

[cwin74]

The proportioning valve idea was a quick thought from zilla along with the vacuum booster. It was part of a quick conversation we had when I met him yesterday. Maybe the valve has air stuck in it. Just running out of things to look at other than re-bleeding all four corners again.

[Jesus_man]

How is the bleeding done? Maybe switch methods and try that?

[Viperwolf1]

What kind of rear disks? Some need to be adjusted before they work well.

[cwin74]

What kind of rear disks? Some need to be adjusted before they work well.

No idea! They have been installed for about 10 years.

Are you sure you have rear Disc's?

100%

[cwin74]

How is the bleeding done? Maybe switch methods and try that?

Had the shop do it so I'm not sure what method they chose.

[ZOSO]

If you have a proportioning valve take it out. Run the front to the front and rear to rear. Put an adjustable proportioning valve in the rear line. Also need to make sure the master cylinder is the right size piston. If the feel of the pedal doesn't change with the motor off you have a bad booster. Pull the hose off the booster and put it on a vacuum pump. See if you can build any vacuum in the booster.

[Viperwolf1]

Pinch either the front or rear hose and see if the pedal feels better. That will point to the trouble.

[Jesus_man]

If the pedal is the same truck on or off, my bet is booster.

[Gunnibronco]

After what I went through last year, I'd recheck and make sure there is no air in the system. I ended up doing lots of extra work fighting a problem that didn't exist. I just needed to bleed more. After chasing my tail for a while, I got frustrated & let a mechanic at our shop bleed the brakes. He gave the truck back to me and said it was done, it was better. I still didn't like the feel of my brakes, and bled my brakes again, and finally got the air out. Changed everything.

Did you run the same Vette m/c before? or did you run a different m/c before? If you just replaced some bad parts, but didn't change the system, I'd look for air first.

I have read that the H-block is not a proportioning valve, and just a connection point for the brake lines. Are you running a specific prop valve? I don't know squat about disk/disk.

[cwin74]

All great thoughts, thanks much. I spoke with the shop that bled my brakes the first time and they said bring it in again so they can bleed them again for free. Maybe I will have them check the booster out too since there is no change in pedal stiffness whether the Bronco is on or off.

[Jesus_man]

See what method they are using. My guess is they have the fancy tool to allow one guy to do the bleeding. Since all of your fluid should be new, bleeding from the caliper up is a good way to get the air out.

Just a shot in the dark - all your bleeder screws are at the top of the caliper right?

[Booger]

...........Just a shot in the dark - all your bleeder screws are at the top of the caliper right?

That can get a lot of people if they did a disk conversion and the bleeder is on the bottom. It takes a lot to get the air out if the bleeder is on the bottom, air travels to the top.

[cwin74]

The shop bled the brake system again using some kind of mystical bleeding tool and the brakes are better, but the pedal is still spongy. They graciously offered to go through the entire brake system inch by inch for a price. I did not take them up on the offer so I'm going to drive it as is for a bit. I'm going to focus on other more fun stuff like suspension, tires\wheel, exhaust, front/rear bumper and such. =)