boggy brake pedal

[texancanuc]

i recently replaced the stock 190hp motor with a 300+ crate, usal upgrades but the cam is mild because i have a th350 trans i know i need the vaccuum for the brakes and i expect lower vaccuum at idle but my brakes seem boggy they work but low pedal also i havent seen any leaks of fluid and the master level is stable i had a real problem earlier with poor quality rebult calipers took 16 to finally get it right

help me guys and gals is it vaccuum related and if yes what can i do about it if not what then???:confused :confused :BOW

 

[lone73]

low brake pedal

I'll bet you still have some air in the lines. If the pedal sinks while your stopped, holding pressure on it I believe thats an internal leak inside the master. Time for another master cylinder. Sounds like you just need to do some more bleeding though. I like to tap the caliper a few times between bleed strokes to jar any bubbles loose that might be clinging to the walls of the caliper. I also hold the bleeder screw closed AFTER the wife starts downward pressure on the pedal, open the screw to get a short burst of fluid out then close it quickly and firmly. Remember to bleed RR, LR, RF, LF.

good luck with it

lone73

 

[texancanuc]

thanx buddy i was gonna put her up on stands this weekend i love my new engine cant wait till its broken in:

as for the master its new <8months old but i have it redone 5-6 times no problems since it seems to have gotten boggy since the new engine was installed thats why i was wondering if it was a vaccuum leak

keep it between the ditchs

 

[Ludigdrums]

bleeding sequence

Hey lone73...I don't know if this is different for your model...but my 81 Shop Manual says:
"left rear inner, left rear outer, right rear inner, right rear outer, left front, right front".

Just wanted to clarify.

I unfortunately am having problems with my brakes as well. I bleed them and they are real good for a while. But after a day or two of driving, the pedal gets real low, eventually causing the brake warning light to come on. I see no fluid at any of the calipers. I don't know where this air is coming from. Hopefully I can find some answers!

- Jeremy

 

[igasper10]

Re: bleeding sequence

I unfortunately am having problems with my brakes as well. I bleed them and they are real good for a while. But after a day or two of driving, the pedal gets real low, eventually causing the brake warning light to come on. I see no fluid at any of the calipers. I don't know where this air is coming from. Hopefully I can find some answers!

I believe your problem is caused by warped rotors. You need to either have them turned, or get the o-ring type calipers. When a rotor is out of spec on our Vettes, the calipers actually sucks in air at the piston.

As for texancanuc:

I recently had a similar problem with my 74. I had new everything (lines, calipers, m/c, etc.) but I was still getting spongy pedal even after pressure bleeding (which is also a must for these cars). It turned out to be my distribution switch that's mounted to the frame rail under the m/c. It may be called a "proportioning valve" on your year, I'm not sure... It had frozen up and was causing all of my brake woes. Now that I've replaced that I stop on a dime! See this thread:

 

[lone73]

Re: bleeding sequence

QUOTE]Originally posted by Ludigdrums
Hey lone73...I don't know if this is different for your model...but my 81 Shop Manual says:
"left rear inner, left rear outer, right rear inner, right rear outer, left front, right front".

Just wanted to clarify.

I unfortunately am having problems with my brakes as well. I bleed them and they are real good for a while. But after a day or two of driving, the pedal gets real low, eventually causing the brake warning light to come on. I see no fluid at any of the calipers. I don't know where this air is coming from. Hopefully I can find some answers!

- Jeremy [/QUOTE]

The RR caliper first is a rule of thumb that says do the furthest one from the MC first and work back. If your manual says LR first then I'd definitely follow the manual. I bleed RR first on mine w/no prob's.

Your prob could be pumping air as iqasper10 stated.
I've never had the air pumping prob with my car and I've had warped rotors before. I don't know why that is. I just have Autozone calipers on my car. Nothing special. I have never heard any other thoeries about the cause of the problem. I have seen some posts that state only a few thousandths of run out at the rotors can cause the problem. I probably have that much all the time and don't feel it when I hit the brakes.

When you bleed your brakes after they get mushy do you actually see air bubbles coming out of the caliper ?

 

[Ludigdrums]

Yes, I do get air bubbles. What seems weird to me is that this problem seemed to develop rather quickly and it's been like this ever since it first happened. Maybe I've just been driving it more...I don't know. So, my point is that I cannot see how the rotors could get warped or have runout. I have read those posts as well, but I don't have the proper tools to measure it. :-(

- Jeremy

 

[lone73]

Air pumping

Ludigdrums,

I wonder if anyone has had the air pumping problem with newly rebuilt calipers or just with old ones. It's possible that one cure for the symptom is to eliminate ALL the run out, but the problem's cause is old caliper seals or something else.

How old are your calipers since the last rebuild?
.
.
.

 

[cwerve74]

I would like to point out the runout problem (air pumping)with brakes can come from hubs and wheel bearings. Although the most common is disk rotors. You need to have the disk trued up while on the car. You need to measure the runnout with the lugnuts on the disk rotor(wheel off).

You can get a dial indicator used for measuring runout for under $20.
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=623

Plus a magnetic stand for under $10.
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=5646

Might be a good investment for the tool closet. You can find this stuff on ebay as well.

 

[Ludigdrums]

Thanks a lot, I will have to get these tools.

The second to last time I bled them, I concentrated on the rears cause I thought they were the culprit. Couldn't get the pedal to raise. So I decided to do the fronts, and then the brakes were awesome. So that led me to believe that it was mainly the fronts. So, after a couple days, the pedal got low again, and I thought, well I'll just do the fronts again. This time, no luck...still no pedal. So, now I'm all confused and will try doing the rears again soon. It's getting so darn cold out...grrr!

- Jeremy

 

[silver77vette]

Had the same kind of problem with my 77 after putting on new calipers,rotors,pads & SS brake lines. Would be fine for a day or so. Even took in it in to a shop.Still had the problem till we did a pressure bleed on it. Worked like a charm. Good firm pedal and haven't had a problem since.That was a year ago. Could also be a lack of vaccum due to the cam. Hope this might help you out! Good Luck!!

 

[Ludigdrums]

Further Help Please...

OK, just a little more information that might help me get an answer here.

AS of yet, I have not been able to check for runout or warped rotors (however, they feel fine while braking...no vibrations or anything).

What I have done, is bled them in the correct sequence using not only a vacuum type Mityvac pump, but also with a helper pumping the brakes.

This was Friday, and the test drive went great...everything felt fine. I convinced myself to trust it enough to take it to NJ, coming from NY. Anyhow, 30 min into the trip, no brakes and the brake light comes on. This was at night and I was able to downshift most of my way, and if I really held down on the floor with the brake pedal, it did stop very slowly.

So, that brings me to earlier today, where I repeated the bleeding process once again, and everything was cool again. Just took another 45 min trip, and I can tell the pedal is almost to the floor...no brake light yet, but a few more minutes of driving, and I'm sure it'll be there.

Here's the thing...I am loosing absolutely no brake fluid from the master cylinder. There is no brake fluid to be found on any of the calipers or lines. If I am sucking air, I cannot see how it is possible to not have any signs of fluid anywhere. Also, I do not feel any vibrations coming from the braking system. I can't fathom how this problem could arise after driving for 30 min. I understand the whole runout issue, but again, don't think that it happens that fast.

Also, how does the brake warning light work? Pressure, pedal travel? Cause mine seems to know when my brakes aren't working.

Another thing...it seems that most of my air is coming from the fronts, however, it seems liek I need to bleed the rears in order for them to work properly again. This I don't understand because the master cylinder has separate chambers for the front and rear.

What about the proportioning valve? What does that do and can it be the culprit? How about the "blocks" on the lines?

Sorry so long people, but all of these circumstances just seem strange to me. Thanks for taking the time to read and reply!

- Jeremy <--poor guy that wants to drive his Vette but can't because of silly brakes!

 

[igasper10]

I'm not exactly sure about your year, but on mine the proportioning valve is what controls the brake warning light. If one system fails (front or back) the valve moves either forward or back accordingly and grounds out the switch for the light to come on. I would say your valve is fine if the light is only coming on when its supposed to. You're sucking in air from somewhere. Someone with more year-specific info will chime in...

ian

 

[cwerve74]

When I got my 74 it would go as little as 10 miles before there was enough air in the front to make the pedal go to the floor. My problem was runout. You have to check the runout with the rotors on the car. Most brake shops do not understand that. Based on your description I would not drive the car untill you get this under control. You can take the front hub , rotor and wheel bearings off the car as single asembly. Then carry it to a brake shop. They should be able to measure the runout.

 

[lone73]

Re: Further Help Please...

Another thing...it seems that most of my air is coming from the fronts, however, it seems liek I need to bleed the rears in order for them to work properly again. This I don't understand because the master cylinder has separate chambers for the front and rear.

What about the proportioning valve? What does that do and can it be the culprit? How about the "blocks" on the lines?

Sorry so long people, but all of these circumstances just seem strange to me. Thanks for taking the time to read and reply!

- Jeremy <--poor guy that wants to drive his Vette but can't because of silly brakes! [/B]

I agree, this is strange to me too. I have NEVER had this air pumping prob and I KNOW I have some rotor run out. For your pedal to go to the floor with no brakes and be caused by air in the calipers I would think it would have to be an AWFULL LOT of air to cause that symptom. If you're getting significant air out each time you bleed then I suppose it must be pumping it in, but why isn't it doing it on my car as well? As I said above, I have just plain-jane Autozone rebuilt calipers on my car, no stainless steel sleeves, no micro polishing, nothin' special but they ARE (or were when I first put mine on the road) rebuilt. They have about 50K miles on them now - no prob's still and it's driven daily.

I still believe this pumping could be related to old, possibly hardened caliper seals. How old are your calipers?

I'm sure, as with most of us poor slobs, money is an issue but if it weren't it'd sure be an interesting experiment to throw on a set of rebuilt front calipers to see if that fixes the prob.

When you sit with constant pressure on the pedal after you bleed it does the pedal very slowly sink down? May have a master prob too.

good luck Jeremy

 

[Ludigdrums]

Thanks for all your replies...

lone73, I think my calipers are all original. They say Delco-Moraine or something like that. As for your second question, no, the pedal doesn't sink to the floor after the bleed.

cwerve74 - did you problem happen gradually, or all of a sudden? Cause that's what I feel happened with me. It had been fine since I had the car, and then all of a sudden, I got into this cycle of bleeding and then the pedal getting soft again.

igasper10 - thanks for the info on the proportioning valve.

- Jeremy

 

[cwerve74]

I had the problem when the car arrived. I think the previous owner worked on it because there were a lot of what looked to be new brake parts. Plus the previous owner said in his car description --"the brakes might need some attention.". So I can't tell you how it started. I can say this--- as I worked to understand the problem (I had no previous experience with this kind of problem) I think it might be a combination of runout size and a speed range. I would bleed the brakes and drive slowly ( less than 20 MPH) and the pedal would stay hard then a short period of 20-40 MPH and the pedal would go the floor go to the floor. Although it is just possible it was the number of revolution of the brake rotor that was important. I was always surpised by how little air it took to erode the braked effectness.

What I can say is I spent some $$$ and all the wheels are now less than .010 in runout and all o-ring seals on SS calipers. Problem solved. I started with replacing parts one at a time. Then had just one too many heart stopping moments with brakes that I believed were fixed/good to have them disapear at just the wrong time. Brakes are important.

 

[JohnZ]

Classic, "textbook" air-pumping symptoms; since it's primarily in the front system, it could also be incorrectly-adjusted front wheel bearings contributing to the lateral runout, in addition to the usual rotor runout and/or hardened seal problems. Re-adjust the front wheel bearings EXACTLY as described in the Shop Manual and see if you still have the problem. Note that there are two cotter pin holes in the spindle (not just one) - one horizontal, and one vertical, so you can get the adjustment very precise, within 1/12-turn.

 

[tazdevl35]

In order to isolate the calipers from the equation you need to purchace 4 clamping tools from either sears or a parts store that sells tools. In a pinch needle nose vice grips will do but BE ADVISED they can damage the hoses if you arent careful. Pinch off each brake hose till all 4 are pinched off then apply the brake pedal(you will need 4 sets of clamp tools or 'grips) and watch for pedal travel. The pedal should be high and hard at this point. If it falls or is spongy there is a lot of air in the system or the secondary seal or quick take up passage in the master cylinder screwed up. If its a good pedal remove each clamp one at a time rechecking the pedal for sponge and drop as you remove each clamp until you remove the one that lets the pedal drop. This is either a bad caliper or it has air in it...which could come from runout or bad seals in the caliper. This tell you which wheel is the troublemaker and the repair should be easy from there.



Taz :cool

 

[Ludigdrums]

Taz, that sounds like a good method to me. This should be done before or after the system is bled properly? What about the tools...I don't want to mess up the lines...could you maybe get me a link to the correct tool from sears or something? That'd be great!

JohnZ - thanks also..I will do this as well.

cwerve74 - yeah, interesting stuff. What I have come across is that it doesn't seem to have much to do with how much or how little I use the brakes, as much as it does how long I drive for. I totally agree that brakes are a must!

Thanks again everyone!!!

- Jeremy