brake problems

[coletrain777]

So, I have been driving my truck ( 71 f100) for a while now and the brakes have always been really soft. The truck is four wheel drum. I recently replaced the front drums and wheel cylinders. Here's the problem... When I bleed the brakes I have great feeling pedal (real firm) when I get in the cab, but as soon as I start the truck, the pedal goes back to feeling spongy. Could this be a brake booster problem. The only thing I can think of is that it would have to be somehow vacuum related if the pedal gets soft as soon as I start the truck. HELP PLEASE!!!
Thanks
Cole

[theogre5150]

That's the way it is with the newer vehicles. You gotta have the key in the on position. Can you see a difference in the on vs off position without cranking your truck?

[coletrain777]

Nope,
There's no difference until I start the engine, which makes me think that it has to be power booster related. I'm sure that I'm not the only one who's had this problem (I hope not). The strange thing is that the brake pedal is really solid and hard right after I bleed the brakes, but as soon as I start the engine... it get spongy. Not sure where to go from here, don't want to just start replacing everything.

[two-bit]

This may not be your booster. This may be your front shoe adjustment.
Any vehicle with power brakes will have a stiff pedal when there is no engine vacuum present.
Add engine vacuum and pedal resistance drops to nill. Thats kinda why they designed power brakes in the first place. (OK no more sm@rt @ss comments).

Does the pedal get stiffer if you pump it rapidly?
How did you adjust your front shoes? Just until you could get the drum on, or after you put the drum on?
You may have to much "play" in your front shoes causing excess pedal travel to get them to push on the drum tight.

Let us know, front drums are a special breed of there own.
Later,
Two-bit

[coletrain777]

Alright,
The pedal doesn't get any stiffer when I pump it. I just went out and adjusted the drums until they had light to medium drag on them when I tried to spin the wheels while in the air. I am not exactly sure how much drag/resistance there should be normally (first vehicle with drums). This seemed to help a tiny amount, but it seems like I have to really push the pedal to make this truck stop, and it gets pretty annoying in traffic when my leg feels like it's going to cramp up. What am I missing here guys, I am pretty mechanically inclined, but I don't have any idea on this one.

[fordman]

when you adjust drum brakes you pin them until they drag and then back them off to where they barley stop dragging. i am unsure of the problem your having my truck does the same thing. i always thought it was the booster going bad. but maybe not. the hard stopping in traffic is a drum brake thing. theonly way to get easyier brakes would be to convert to disc brakes i think.

[two-bit]

When i set my drum brakes on the front, i mark the front tires with a grease pencil (yellow), and watch the rotation.
I usually stop adjusting when i have enough drag to stop the tire in one rotation. The trick is to spin both tires with the same force so i get equal adjustment. Some people have told me i set them to tight. I don't know if thats true or not, but i like the feel of the pedal.
A really hard pedal maybe a bad rubber line also. But since you have lots of travel, then pedal, that might not be your problem.
Check and make sure your master isn't leaking into the booster.
You will never get the brakes bled if thats happening. Look for a wet spot where the master mounts to the booster, also pull the vacuum line for the booster and smell for brake fluid.

Good luck

Two-bit

[Mooosman]

In my experience, if you have a booster going bad, the pedal will feel really good when you first step on it, then as you hold it will gradually go to the floor (engine running of course). From what you describe, I don't think this is your problem.

Certain cars seem to have spongier feeling brakes than others. GM trucks get my vote for mushiest-feeling brakes.

Nick

[67heavystepside]

sounds like natrure of the beast. i adjust my drums a little on the tight side, and let the adjusters do the rest, if your adusting hardware is all in good shape and working correctly, all it should need is a periodic adjustment.


also check the adjustment on the rear breakes as well, that will affect the feel

[l1k2gadd]

Good observation!! Like 67heavystepside said, don't forget to adjust those rear brakes too! You don't want the front brakes doing all of the stopping for your truck. You may find that your pedal pressure changes when the rear brakes are adjusted too.

[QC]

My guess is that if the pedal is spongy there is still some air in the system. All the booster does is help you apply the brakes. There is a solid rod going right through it into the master cylinder. You may have to take it in somewhere to get it power-bled.
If the master cylinder was replaced and wasn't properly bench bled, you can get so much air in the system power bleeding is about the only way to get it out.