Ok, does the brake booster actually have anything to do with stopping power or is it only for a power assist and theoretically just makes the pedal easier to push?

The reason I ask is that I installed yet another master cylinder and my brakes are still not very good. They will not slide any tire on pavement.

What else should I try? Could a new booster fix it? I want to get 2 piston calipers but I don't see the need of spending $350+ if they wont fix it.

Its really only power assist. Why do you think your brakes aren't working? Vacuum? Or something else? I run a relatively radical (for a 454) cam and when I first got it in, I had to tweak the idle of the motor and a few other things, otherwise I was almost sliding into the garage at my house at off-idle RPM's trying to stop the car because I didn't have enough assist. I did get it to work w/o having to switch to manual brakes. With a 233 @ .050 cam I could always lock the fronts (this is before I got the baer 2 piston front system).
I think the point I am attempting to make is really that if you have a cam with very poor vacuum (imho, it takes less vacuum to run a booster than I see alot of people say) you are better served to run properly adjusted manual brakes.

its a 355 with just a 268 comp cam, very mild.

the brakes will not lock or slide the tires, stopping distance from 50 can be as much as 3-4 car lengths at times when it's bad

I'm at a loss for what to try...its like when you first hit the brakes they work but then quickly fade to a point then hang there, they dont fade completely away

what else could it be?

wheel cyliders? calipers?

rotors are aftermarket slotted rotors, pads are thermoquiets

Did you bench bleed the master, then bleed the entire system completly? Do you have any leaks?
Let us know how much vacuum you have available.

Mike

Did you change from drums to discs? Maybe you still have a residual valve in the line for drum brakes. Disc brakes take more pressure to operate properly. Just a thought

When you say they fade... there is no pedal movement occuring, correct?

The function of the booster is two-fold: it is for stopping power and it is also for power assist. There is redundancy and fail-safe built into the system to prevent total failure. Rather than total failure, systems like power brakes and power steering are designed to gracefully (?) degrade as parts of the system fail. Power steering is probably a good example here as we all know we *can* still turn without it. But we also know we will not turn as fast nor anywhere near as easy (and for the physically limited possibly not at all). Brakes are very similiar, utilizing such things are dual piston master cylinders, seperate fluid circuits, and even the boost designs.

It sounds as though you start out with vacuum boost and then the vacuum fades leaving you without boost. You perceive this as fading brakes because the system pressure decreases. You are pushing the pedal just the same but without the extra from the booster the master cylinder sees less. You may only be able to push so hard, after which you are dependent upon the booster to generate additional pressure.

Part of the challenge here is that you are somewhat shooting in the dark. If you had a brake pressure guage you could verify the pressure drops. With a vacuum guage in the right spot you could verify that vacuum drops. But without any way to measure you are left to reason and replace: guess what part is broken and replace it until you find the right one. Under this approach I would start with the cheapest first.

I would check how much vacuum you have to work with. I would check out the vacuum check valve to make sure it is functional. If these cheap fixes don't work, then I would probably consider a booster.

Steve

I agree with Steve. I read some of your other posts and it sounds like you are loosing vacuum. Easy enough to check. Pump the brakes with the car off and the pedal should get real hard. Keep your foot on the brake, and start the car. The pedal should sink. Shut the car off with your foot still on the pedal. If the pedal starts to rise, pushing back up at your foot, there is a leak. Possibly the check valve is bad, missing gasket between booster and master, or cracked booster. The pedal should hold vacuum until you pump it again.

You should also be able to start the car for a minute and let it run without touching the brakes, then shut it off. Wait a few minutes and then remove the check valve from the booster body. If you dont hear the vacuum suck air in, there is a problem in the valve or booster, or missing gasket.

If after pumping the vacuum out and you start the car and the pedal still doesnt move, you are not getting vacuum to the booster. When it does move, you should get three good pumps with engine off before it gets hard. If it only takes one, and it passed the leak test, you might not be getting enough vacuum. Restricted line, check valve, or rear of booster not open to air (wrong boot).

Thanks guys, that does indeed seem like the problem...the pedal goes about half way easy, then gets hard as a rock but still moves just a lil bit.

I'll check for vacuum and the check valve next trip home.

Yes I did switch from drum, I added the booster prop. valve and the valve over the rear axle from the 70 donor car.

If nothing else...I might have shiny new booster to dress up the compartment when I'm done graemlins/clonk.gif

I'm out of ideas to try

I went to Oreilly's and bought a check valve and put that on.

That made a noticeable difference, so I am pretty sure I'm on the right track now. I'm gonna take the time to look for vacuum leaks everywhere I can and if I can find no more, I'm gonna buy a new booster.

This booster is the factory one off a 70 Chevelle so it is very likely it could be deteriorated internally or something.