medicman

Now this is wierd I'm driving along start coming up on my stop slow down and come to a stop but the brakes will feel hard for a sec as soon as I apply them then they will go to the floor. but the wierd part is when its stopped and were sitting there idling with my fut on the brake it kinda motorboats if you will while idling. It will idle and every few seconds the rpm's will drop and then pick back up. Now this only seems to happen while I'm holding down the brake as soon as I let up off the brake it resumes idling correctly so what the heck causes this?

millball

Sounds like at least part of your problem may be due to a bad brake booster. (Brake pedal affecting idle speed) Even so, the brake pedal should'nt go to the floor. You may have other issues with the brake hydraulic system as well.

medicman

would the brake booster being bad cause the idle to change when I'm at a stop and the brakes applied and it idling? It goes up and down when I have the brake mashed I let up and the idle goes back to normal.

medicman

How can I check the booster to see if it is bad?

millball

Yes, a ruptured brake booster diaphram might cause this.

I suppose that there might be other causes that could change your idle as well.
You got auto or manual tranny??

You might test the booster by applying vacuum to it with a mighty-vac type hand pump while the brake pedal is depressed by a helper. You might have to pump quite a bit, but ultimately, vacuum should build in the booster and not bleed off.

Air leakage thru a ruptured booster diaphram can definitely cause the idle fluctuation when you depress the brake pedal..

medicman

Yea its a standard. Like I was saying when I'm trying slow down I have to pump the brakes or at least let off of them and reapply them till it stops cause if I just mash the brakes they will seem hard for a sec then as I continue to hold them they bleed off I guess cause the pedal will continue on to the floor. once I come to a stop and just sitting there idling with my foot on the brake thats when the idle starts motorboating if you want to call it that it goes up and down.

millball

Like I said before, you might have one problem on top of another one.

Hows the fluid level in the master cylinder??

osv

good call about the booster being a potential problem... is the idle speed changing as well?

since there is a check valve in the vacuum line going back to the engine, a good booster will retain it's vacuum after the engine is shut off... so you can pull the vacuum line where it hooks up to the body of the booster, and you should hear the air rushing back into the booster... it makes a pretty loud noise.

millball

Even a bad booster will often hold vacuum when at rest.
The truth is told if it will hold vacuum when the pedal is depressed.

medicman

yea the idle speed changes as well. fluid in the resavoir is full,so whats the easiest way to make sure my booster is okay or if its the culprit?

osv

i tested the check valve and the hose, at the minimum... pumping the booster directly with a mighty vac won't test those items.

granted, those items don't usually fail... but there has to be an easier way than pumping by hand, so i looked it up.

turns out that you can use engine vacuum instead of hand pumping, which i did not know how to do... see the "brake booster operating check" here, about page 9:
http://www.autoshop101.com/forms/brake05.pdf

apparently it's normal behavior for the depressed pedal to sink when vacuum is applied... I've noticed that behavior many times, but never asked why :-/

millball

Excellent treatment of vacuum booster operation and diagnosis there OSV.

Brakes should still function though with increased effort, even with non-functional booster.
They should not go to the floorboard if the hydraulic end is sound.

Gamefreakgc

There's only two parts to the braking system that applies pressure: fluid and the brake booster. If fluid levels are good, then it's your brake booster. I second the pressing the brake when the boost hose is removed. There's really not much else to it.

Also, you could test it with the engine off. Pump the pedal a few times and it should firm up so that it doesn't even move (except for a little bit of a wiggle). If this isn't the case, definitely your brake booster.

medicman

Okay I really appreciate all ya'lls help, I havn't checked much since it happened yesterday eve however I did go out and pull it into the sho this morning and before I started it I mashed on the brake pedal and it was hard as a rock pumped it a few times never even acted like it was gonna go down stayed firm the whole time until I start it then it starts so I dont know if this means my booster is fine but it seems as though if the booster was bad it wouldnt do this.

CJM

it is possible the master is internally leaking between the ports.

medicman

would there be some kinda way to test it to see I hate to run out and buy a booster if that aint it cause them dudes aint cheap but if I got to I guess I will.

millball

If you read the link that OSV posted, it tells you how to test it.

osv

there are several booster tests in that link I posted.

Ven0m

Couldn't this have to do with air in the system as-well? setting aside the rough idle while using the brakes, the pedal going to the floor indicates to me air in the system..i drove a 68 caprice just the other day and after a little work on it the pedal went straight to the floor. after pumping it feels good but it goes away after a few presses. as soon as the brakes were bleed the problem was solved..I had another experience in an Acura integra where the brakes failed going down hill doing 60 this being my ex- girlfriends car i pumped the brakes and it worked just fine for a little bit..went back to her house bleed them for her and they worked perfect.

Cmayer

Just a guess, but if there's a vacuum leak and your idle has increased because of it, you could be hitting your fuel cut off which happens when you have too high of an idle and you're on the brakes.

Is your overall idle higher than it was prior to this problem?