Bad brake booster?

Topic: Bad brake booster? (Read 4793 times) previous topic - next topic

Bad brake booster?

August 16, 2010, 08:35:32 PM

Is there a way to tell if a brake booster is bad? When I pull the main vac. line, and hold my finger over the line, the brakes work like a charm, when I hook it back up, they go all the way to the floor, with no stopping power, and under the steering wheel by the brake booster sounds like a definite leak of some sort. This car is an 87 grand marquis BTW

Bad brake booster?

Reply #1 – August 17, 2010, 01:29:40 AM

Usually if the booster is bad you'll have little if any power assist (ie the pedal is really hard).

Bad brake booster?

Reply #2 – August 17, 2010, 08:36:56 PM

the tbird i bought last year has a bad booster and the same symptoms you describe. you can hear it hissing by the pedal when you use the brakes but the car still stops, the pedal doesnt sink but has little resistance. its definately not that vac thing for the parking brake. i have no idea how to tell if that thing the vac line attaches to the booster is bad or not, or even what its called specifically. too tired to guess :/
i wouldve guessed the master cylinder was to blame at first if not for it being a new replacement.

Bad brake booster?

Reply #3 – August 17, 2010, 09:20:18 PM

Quote from: Ductape91;332344

the tbird i bought last year has a bad booster and the same symptoms you describe. you can hear it hissing by the pedal when you use the brakes but the car still stops.

I had air noise when pressing the brake pedal but apparently it's still there after installing a new booster. All hoses are new and it's not coming from the cruise control. Everything works great. I think the noise I hear is just the air moving around in the booster so I don't think hearing any sounds necessary means anything bad.

My old booster did have problems pressing the master cylinder's pushrod though - just swapping out the booster made the brakes great. Both had the pushrod adjusted to the same length/clearance, yet the lack of brake power when pressing the brake, and reaching the floor without lockup meant something sure was wrong. The pedal did get hard 2/3 down but the last bit of pedal did nothing more.

How to check if they're bad? No clue. I'd assume it'd have something to do with checking the vacuum from the port and watching for leakage or removing it and checking for proper sealing between the inner and outer areas that should be sealed off from one another at all times - the rear area either having vacuum or atmospheric pressure depending on its position. Then there's the return spring but this shouldn't impact breaking ability, just pedal return.

Bad brake booster?

Reply #4 – August 17, 2010, 10:01:32 PM

Well the brakes go all the way to the floor with almost no force at all and stopping at around 30 mph seems nearly impossible, and the brakes do not lockup when it hits the floor. I have no clue. I replaced a caliper with a frozen piston, and the brakes were ok for about two weeks, then seems like they went out almost completely.

Bad brake booster?

Reply #5 – August 17, 2010, 11:18:18 PM

Re-bleed the brakes. If that dosen't help, my bet is a bad line or bad master. My dad got three bad boosters from autozone before one worked.

Bad brake booster?

Reply #6 – August 17, 2010, 11:38:23 PM

Quote from: Haystack;332376

Re-bleed the brakes. If that dosen't help, my bet is a bad line or bad master. My dad got three bad boosters from autozone before one worked.

Not being able to stop well at 30mph would point me to contaminated fluid (old - replace it), air in the lines, or a messed up master cylinder. Other reasoning such as soft brake hoses, pistons, and booster I would look at last. Have you jacked it up and checked to verify that pressing the brake pedal actually engages the front brakes? You could be getting just rears - that'd cause severe braking problems also.

*edit*
Cut out some stuff as it wasn't really relevant and just sounded completely...wrong when I read it back. I have no idea why replacing my booster helped my brakes, with no other changes, with a proper pushrod adjustment on both, but it apparently CAN be the booster, although unlikely.

Bad brake booster?

Reply #7 – August 18, 2010, 12:47:47 PM

With the engine not running you should have a full pedal and it should just be rock hard. Typically you bleed the brakes with the engine off and a buddy helping by pumping up the pedal and then you opening the bleeder at the caliper or wheel cylinder.

Before you do anything check and see how the pedal feels with the engine off. You should have minimal travel (inch or less) and it should get rock hard. Have a buddy check the fronts to see if the front calipers are engaging and check the rears to see if the calipers are engaging. If you have drum brakes see if they can hear the rear brakes moving and make sure they are contacting the drums by spinning them. If all of this is good then bleed the brakes. Start on the left rear as it is the farthest away from the master cylinder (MC). I would suggest you completely cycle the brake fluid at each brake i.e. until you get new, clear fluid with no air bubbles. Clear tubing into an old brake fluid bottle or something similar is great for this. Be sure you constantly check the MC and add fluid so that you do not suck air back into the lines. After you do this try the pedal again. If it goes to the floor or the pedal is mushy then replace the MC as it is most likely bypassing around the internal seals. Bench bleed the MC before you install it as it will make life much easier. You will again have to bleed the brake lines...all kinds of fun and you now owe your buddy a twelve pack of his/her favorite beverage and preferably cold.

As thunderjet302 indicated the booster is just that, a booster. It allows more pressure to be exerted on the brakes than you could normally apply with just your leg power with the lever principle of the brake pedal. The brakes will stop the car without the booster connected to the vacuum supply but with it connected it will greatly increase the performance of the braking system.

Again, this is all based of my experiences with my cars and it is how I trouble shoot braking systems. I also use a power bleeder so I can bleed the brakes in case a buddy is not available. If anyone is interested you can get one here:

http://store.motiveproducts.com/bleeders-c15.aspx

Darren

Bad brake booster?

Reply #8 – August 18, 2010, 06:22:04 PM

Quote from: 20thanniver-ls;332359

. I replaced a caliper with a frozen piston, and the brakes were ok for about two weeks, then seems like they went out almost completely.

thats the kinda thing you shouldve mentioned in your first post

the last caliper i bought from the local autohole came with copper washers that were too thin and leaked after a few days(even properly torqued). had to go to the dealer for them to get the right ones. an old head chrysler guy pointed that issue out to me.

Bad brake booster?

Reply #9 – August 18, 2010, 09:01:57 PM

Thanks for the replies guys. This is actually my brothers car and I'm trying to keep him (as well as others ) safe.

I did check all the calipers/drums to make sure they all stopped the wheels from turning, thats when I notice3d one was bad. We're gonna re-bleed the brakes and put all new fluid in there, and replace both front soft lines, and go from there, prob. a new MC as well, as it prob needs to be changed anyway.

Bad brake booster?

Reply #10 – August 18, 2010, 09:32:22 PM

Quote from: 20thanniver-ls;332528

We're gonna re-bleed the brakes and put all new fluid in there, and replace both front soft lines, and go from there, prob. a new MC as well, as it prob needs to be changed anyway.

Run a hose from the bleeder into a container, down to the bottom, with a little fluid in the bottom. Keep the end of the hose submerged and it shouldn't suck any air back in at any point. Even with two people, I'd still do this although it can also work more slowly when going at it solo.

Can anyone explain why my brake issues went away with a new booster? My brakes were going bad over time, both before and after I replaced everything in the vehicle, but the booster. Replacing this last part made the system work better than newer vehicles I drive (the additional assist helped as well). I got improved brake pedal feel when going to a larger master cylinder and 73mm front/43mm rear calipers. This improvement may have been only from a properly setup pushrod but the system's performance did continue to drop and get near the floor without locking anything up at the end. What is the internal construction of the booster like? Is it possible that whatever is between the pedal rod and mc pushrod could have had an issue? Is it metal or some other material? I just want explanations that no one can ever give me for problems I have with the vehicle that shouldn't exist! So frustrating

Bad brake booster?

Reply #11 – August 18, 2010, 10:17:03 PM

I have seen brake boosters fail alot. I just did one on a 2010 Econoline. My bad was that when I replaced the booster I had the same problem with pedal to the floor and no stopping. Turns out Ford now puts an o ring between where your master bolts to the brake booster, and the factory forgot to put it in....I didn't even know they started putting a o ring in that spot.

But I have seen the diaphram fail and cause a leak inside the car. You hear a rush of air sound under the dash. Seems to come out where the brake push rod inside the vehicle meets with the brake pedal. Don't know why or how it happends but I usually just replaced the booster and go.Probably just a tear in the diaphram since it is just a couple rubber pieces with either pressure or vacuum on one side.

Bad brake booster?

Reply #12 – August 19, 2010, 09:12:18 AM

This was pretty cool, even has and animation on the third page:

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/power-brake.htm

Darren

Bad brake booster?

Reply #13 – August 20, 2010, 01:17:01 AM

Its the booster if it changes with the vac line off. If it didn't, you would have no power assist. Now you have too much if it still works fine (although hard) with it off.

Bad brake booster?

Reply #14 – August 20, 2010, 04:20:43 PM

Were (well I'm ) in the process of replacing the master cylinder, the two front soft brake lines, and doing a full bleed. I broke a steel line last night, of course it had to be the one on the passenger side...... So I called it quits yeasterday after rebending the new line and installing it. Here in about an hour or so (waiting for the temps to come down a little bit) I will be replacing the driver's side brake line and bleeding them. We will see what happens....