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Spongy and low brake pedal

17K views 22 replies 6 participants last post by  Spang 
#1 ·
I have too much travel in my front disc one piston calipers, making brakepedal spongy and low.
I think my calipers is not parallell to the disc anymore, it seems like the outside pad sits a bit wedged so to speak.
When I release the brakes it is air between the disk and outside pad on the little diameter end, but full contact on the big diameter end.

Is this a known problem, and is there a quick fix to it?

When I clamp off the two hoses to the calipers the brakepedal sits very high and is rock solid.
Take either one clamp off and pedal goes too deep towards floor and is too spongy.
Take both clamps off and it gets worse.
The brake warning-light will not go on and pedal will not bottom out, but it is close.

It is a –69 Chevelle powerbrake front disc/rear drum.
I have worked on this issue two weeks now.
Bleeding brakes, changed and bench-bleed mastercylinder a few times, checked all hoses, bleed, bleed again and so on.
Not funny anymore.

A few years ago I had big problems with front brakes sucking in air thru the metering or hold-off valve mounted on the master cylinder.
I just deleted it and brakes was fine again.
Apart from that I think everything is standard Chevelle –69 on this car.
 
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#2 ·
Bleeder screws on the TOP of the caliper, not the bottom?

Any chance you have low-drag calipers on the front? If so, they MUST be used with a quick-take-up master cylinder, or you'll have a low brake pedal.

Caliper mount bushings and O-rings still in acceptable condition? The bushings are brittle--will shatter if abused. Should be two bushings and four O-rings (two large, two small) per caliper, on the bolts that go through the steering knuckle/caliper mount.

Wheel bearings properly adjusted? Too much wheel bearing play will knock the pad and piston back into the caliper body.

Get another holdoff valve. Your front brakes will last longer.
 
#3 ·
Yes, bleeder screws on the top.

I changed to new brake calipers a few years ago before I found out it was a faulty hold-off valve then.
Symptoms was almost identical as now.
Clamp the hoses - pedal ok - take away clamps - pedal to floor.
Same casting number on old and new calipers.
Brakes has almost always been good, exept when the hold-off valve was faulty.
No problems locking all four wheels at 60mph, rears requiring slightly more pressure on the pedal to lock.

Bushings, o-rings, pins, bolts, hoses looks good.
Was replaced at the same time as calipers.

Wheel bearings properly adjusted.
Had a mystery noice coming from the front wheels, everybody thought it was wheel bearings.
But it did not go away until I changed front wheels...

I picked up my old calipers today and thought i would give them a try again and see if that helps.
But I did not remember I took them apart when I changed calipers last time.
 
#5 ·
Denny.
You are correct about my brakes feel just like i have low-drag calipers.
I have spent several hours on this Chevelle-forum, searching for an answer.
I found this link a few days ago, never heard of low-drag calipers before.


A few years ago I ordered new/remanufactured calipers to my Chevelle –69 and got same castingnumbers on them as the one sitting on the car then.
Brakes has almost always been great on this car.
This makes me belive I have the right calipers now, but who knows for sure?


It can be air in the calipers.
I have bleed them multiple times.
Gravity-bleed, pressure-bleed and vacuum-bleed.
Also have pushed pistons back in caliper a few times, just like when you change brakepads.
Two big bottles so far, 2 Litres (2quarters).
I am pretty convinced it is not air left in the calipers, but again who knows?


It can also be too much movement of the piston for some reason, making it act a bit like low-drag calipers?
The calipers is self-adjusting to aligment, but I start belive it is something wrong here making piston travel too long.


Is there anything I have overlooked so far?
It is only brakes, how hard can it be to fix!?
 
#6 ·
Have you tried adjusting out the rear brakes?
I've had spongy brakes until I adjusted out the rears before.

As for the crooked disc to pad, does the car shake when brakes are applied?
Remove, clean and ispect the inner races on the rotor. Make sure they haven't started to pull out and get crooked. Inspect the bearings while you have it apart. The rotor could be pushing the caliper piston back, but you would feel a heck of a vibration when braking. If the braking action is smooth, I would rule that out.

Make sure the caliper mount pins are parallel with the spindle when installed.

Did this problem just suddenly come up? Or did you change something and then this started?

I would start by replacing the hold off valve. Then inspect the calipers really well, to rule out the system sucking in air.

Bleed the master cylinder on the car. Push the pedal in, and then crack the line. Tighten before you let up on the pedal, then repeat. Could be a failing master cylinder...

Good luck and keep us posted.
 
#7 · (Edited)
Dan.

I thought this issue gradually has getting worse over time, but I am not sure anymore.
It might be the owner (me) doing something wrong earlier or now.

I do not have the hold-off valve installed.

Rear brakes adjusted properly.

No shake when braking, wheel bearings ok.
Actually the car is on jackstands now, wheels removed.

I have tried a new and the old used master cylinder, bench-bleed it but I have not bleed it on the car as you describe.
I will try that.

How can I check that caliper mount pins are parallel with the spindle when installed?
Is it just to look at it, or is there a good way to measure it?
 
#8 ·
I tried to use C-clamps today, holding caliper pistons in the all in position.
Just like when you change brakepads.
I tried it to test the whole complete brake hydraulic system, eliminating any movement in the calipers.
No improvement, pedal still goes almost to floor.

I am pretty sure I do not have air in the system from the master-cylinder to the hoses.
But it seems to me now I have air still trapped in the calipers, what else could it be?
I will take calipers loose, hoses still connected.
Bleed and at the same time move calipers in different angels to get all air out.
 
#9 ·
Have you replaced the brake hoses to the calipers? Sounds like they are ballooning somewhere... You may not see them actually ballooning, they do fail within the hose liner and you may never see it. Check the rear one too.

If you've clamped off the calipers at the piston and you are still getting spongy brakes, then you have eliminated the calipers or any air stuck in them as a problem.

Now you move onto clamping the rear brake hose now. While still having the front calipers clamped. Try to clamp as close to the master cylinder as possible.

If the problem goes away, then its in your rear brake system. Bad wheel cylinder sucking air, or a flexing rear brake hose, or your rear brakes way out of adjustment.

If it still goes to the floor, is either a bad master cylinder or a bad brake hose.

Don't replace with old, known bad parts. Replace them with new ones if you can. And while you are at it, replace that hold off/proportioning valve. It'll keep you from locking up the rears and crashing that freshly fixed car!!:)
 
#10 ·
Now I have changed almost all parts in the hydraulic system to the front brakes.
New mastercylinder, calipers, bushings and o-rings, hoses and brakepads.
Also eliminated the brake warning light block, bypassing it using tee fittings instead.
I am not using the hold off/proportioning valve at all.

I checked the push-rod length, and it seems to be ok.

NO IMPROVEMENT at all, I am totally lost on this issue now.

I have read a trouble-shooting section suggesting a bad power brake can give excessive pedal travel on Mopar cars.
Is this possible on GM cars too?
 
#11 ·
When I release the brakes it is air between the disk and outside pad on the little diameter end, but full contact on the big diameter end.
Photo, please.

I checked the push-rod length, and it seems to be ok.
HOW did you check pushrod length? There are TWO pushrods--one from pedal to booster, one from booster to master cylinder. The one from pedal to booster should be in the LOWER hole in the brake pedal.

When you have a helper push the brake pedal (less than one inch) there should be TWO "geysers" --one in each of the reservoirs--as fluid is pumped out of the compensating ports of the master cylinder.

That assures that the pushrods are not too LONG.

If the pushrod(s) were too SHORT, one inch of brake pedal travel might not provide any movement at the master cylinder, and you'd have no geysers until the pedal was pushed farther. Or, you'd have problems adjusting the brake light switch because the pedal was already too low.

Any chance you've got a master cylinder with a deep pushrod socket, and a booster/booster pushrod that's intended for a shallow pushrod socket master cylinder? You'd maybe have an inch or three of brake pedal travel that provided NO stopping power, then you might feel the brakes engage as the pedal dropped farther. What brand of booster do you have, and what brand of master cylinder? Delco master cylinder goes with Delco booster, Bendix master cylinder goes with Bendix booster. PHOTOS would be welcome.

Schurkey's Rule Of Thumb for brake pedal height: The brake pedal, on a NORMAL, ORDINARY GRADUAL STOP should STILL be HIGHER than the gas pedal in it's "idle" position. Level with the gas pedal is somewhat acceptable. Lower than the gas pedal is unacceptable. The difficulty here is describing a normal, ordinary stop.

I have read a trouble-shooting section suggesting a bad power brake can give excessive pedal travel on Mopar cars.
Is this possible on GM cars too?
Not that I've ever witnessed. I'm prepared to learn something here, but so far as I know, a defective booster can cause a HARD pedal, but not a LOW pedal. The pedal may drop a little bit as vacuum is applied to a booster that's been drained of vacuum--but the pedal travel isn't that much.
 
#12 ·
I checked pedal to booster pushrod length by loosen the clevis at the brakepedal.
Pushed pedal to stop full-up position.
Was approx 1/16 free play.
Lower hole on the brakepedal, upper is used by the brake light switch I think.

I also tried to measure the pushrod length comming out of the booster.
It was difficult to measure, but it is within + or - .020” compared to my deep bore master cylinder.
 
#13 ·
I don´t know how to post photos here, do I need to upgrade my membership?

I will try to explain it in words instead.
When I release the brakes the outer pads had contact with the disc at the outer diameter.
On the inner diameter I could insert a .016 feeler gauge between the pad and disc.

I think I have a Delco-Moraine power brake booster according to my overhaul manual.
Non-adjustable push-rod in booster.

My mastercylinder looks like this one.
It has 11/8 bore
http://www.summitracing.com/parts/DHB-M89160/?rtype=10

I will do the “geyser” test.
 
#14 ·
I don´t know how to post photos here, do I need to upgrade my membership?
No. Upload the photo to a hosting site. I use my own web page provider, but Picassa or any of the dozens of other sites--OR--the Showroom feature of THIS web site will also work.

Once the photo is on the 'web, you use the picture icon in the text box for typing replies--it's the postcard-looking icon (two mountains and a postage stamp) in yellow and grey. Add the link to your photos in the drop-down box, and click "ok".

My mastercylinder looks like this one.
It has 11/8 bore
http://www.summitracing.com/parts/DHB-M89160/?rtype=10

I will do the “geyser” test.


I'd have expected the other style. More like this one:

but as long as it is a proper match to your booster...I guess...it should be fine.
 
#15 ·
Today we did the “geyser” test.
It is two geysers appearing directly when brakepedal is depressed.

I can hear a “hissing” sound when the brakepedal is depressed.
But I can not tell where it comes from.
It can be from behind the mastercylinder, the gasket between mastercylinder and power brake.
Or the reservoir cover.
Can it still be air in the mastercylinder making this noice?
I will crack the nuts holding the brakelines open when pressure-bleeding to get rid of all possible air.

I may try posting a photo in a few days.
Seems to be many small steps to get it right.
Take a photo, transform it to the computer, use the right format, upload it somewhere and post it in this forum.
It may take as long as it takes to fix my brakes, 3-4 weeks now?!
 
#18 ·
If I clamp off the hose to the rear brakes or apply the parking brake nothing changes at the brakepedal.
I would say it is way too low but quite firm, not spongy.
It appears to be soft due to too much travel in pedal before it builds pressure.

If I clamp off both front hoses brakepedal is very high and very firm.
Take one clamp off and pedal goes too low, take both off and pedal goes way too low.

I can not get my foot in under the brakepedal when applying brakes when engine is off and powerbooster is without vacuum.

I did a testdrive and tested the brakes.
When driving slowly and braking slowly everything feels ok.
Brakepedal is not depressed deeper than normal.
But when you drive faster and apply brakes hard, it for a second or so feels like the rotors where full of oil.
Pedal goes almost to floor and then the front wheels locks up.
Push a little more on the pedal and I think rear wheels locks up right after front wheels.
This test was done on dry asphalt at 35-40mph.

This is a street/strip car, 3950 lbs raceready.
127mph at the finish line has not been any problem before.
 
#19 ·
Now I have taken off the power-booster and only using manual brakes instead.
This move was something I had planned to do in the future, but now it was time to test it.
And about my question if power-booster can make the pedal low, I can not see this can happen at all.

I also did check so the calipers are parallel with the discs.
I could measure a difference at .040
This would not be an issue using stock floating calipers I guess?

I made two 2in. thick “brakepad spacers”.
Took calipers off front discs, hoses still connected.
Turned them upside down and installed my spacers instead.
The spacers replaces the disc and brakepads, eliminating any misalignment at the caliper/disc/brakepads.
The pedal was rock solid when using the spacers.

Took one spacer off and installed the caliper and brakepads on the disc.
Pedal goes too low again.

Then I did check my new brakepads, and they where quite crooked from the China factory.
Thought I could have found my problem now, but new good brakepads did not solve it either.

I let the car sit a few days having both spacers installed.
Then I tested the pedal and it is again too low.
Could not get the pedal to be firm by pressing it several times.
Then I used a pair of pilers to clamp off the hoses again, and the pedal is rock solid.

No air in calipers, and have been tested two pair of calipers.
Same results.

What will my next move be?
Any ideas?

See pictures below!?
My mastercylinder and brake booster.
Brake check.
 

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#20 ·
Your problem seems centered around the lack of parallelism of caliper to rotor.The pedal travel is correct with your blocks in place of the pads but excessive with all parts installed and road ready. Could your caliper mount brackets be at fault ? you have swapped calipers around enough to eliminate them and checked the wheel bearings and rotors.A bent spindle is another possibility ...but both sides bent the same ?....not likely. Maybe you could shim the caliper mounts slightly to better the alignment .
 
#21 ·
I had a friend here today.
He is a proffesional mecanic and has been dealing with Chevys and other cars for 20 years.
He said it is not unusual to have to strighten the caliper mounts sometimes.
But mine seems to be just fine.

We also looked at the calipers.
Had the spacers installed and watching the calipers when braking.
They seems to flex quite a bit during hard braking.
We do not know if this is normal, but can I have four too soft calipers?

To make a long story short, he suggested to install my new, stright brakepads and drive the car 125 miles and see if it helps.
If not, he suggested to install new Delco discs and brakepads.
He did not believe my calipers where bad, but who can be sure?
Or buy a complete kit whith bigger discs, calipers and so on.
 
#22 ·
Spang, with 46 years in the Automotive Repair field, there are some front brake pads that are spongy feeling until they are properly bedded, that means that the vehicle will need to be driven for a couple of miles while applying the brakes many times, but not actually stopping at any one time til you get back to where you are working. Just an FYI, Wagner pads seem to be the worst ones for this. Do drive where you can without impacting traffic if at all possible and good luck and report back here.
 
#23 ·
I converted to manual brakes still using the bigger 11/8in mastercylinder instead of the smaller 1in that is often used with manual brakes, just to get more fluid from it and get a higher pedal.
Pedal is adjusted as high as possible.
When measuring at the pedal I can depress the pedal 4.5in now, not using too much force.
(It is not two foots on the brake and cramping the steering wheel.)
I still think this is too much, what du you say?
And what will the travel be if I install a 1in mastercylinder?

I also installed new brakepads, Raybestos PGD52M.
This is the same I always have been using, and also the one that was installed when my brake problems started.
And this is not the crooked China one I tested at one point.

Took the car on a test drive today and I have to press the brakepedal much harder now at low speeds, when not using the brake booster.
Did a few tests and it is now pretty easy to lock all four wheels at 50-60mph!
I would guess I have to depress the pedal 3.5in to do it.
At light stops I do not think I have to depress the pedal more than ½in now.
It is easier to “feel where the brakes is” at hard stops, but much harder to press at light stops, compared to when using a brake booster.

I did drive a few miles, but I think my brakes was ok from the beginning this time.

I still do not know what the reason to my brake problems was in the beginning.
And what move it was that cured it.
I cross my fingers and hope my brakes are ok now, time will tell.
 
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