Couple brake issues with the '69.

69427

The Artist formerly known as Turbo84
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Clinging to my guns and religion in KCMO.
Got two items making me scratch my head. It seems like every time I bleed/flush the brake fluid I have a nice firm, short pedal, but as soon as I start the engine I lose (pedal moves closer to the floor) about an inch of travel before the pedal gets firm. The car still stops fine, but it's not a warm fuzzy feeling having that low of a pedal when coming into a corner rather quickly. (I make no claim to have been able to remove 100% of the air in the system, so I am not discounting that as one possible cause.)
A new additional item I'm getting is a very audible click in the RR area of the brakes once the pedal goes down close to the floor. I also can feel the click in the braided line feeding the RR caliper. I initially entertained the thought that the noise was coming from the combination valve, but the instrument panel brake light doesn't come on, and I haven't been able to run a long length stethoscope to the combo valve to eliminate it as a possibility. (I had the wife stomp on the brake pedal when I was able to track the click noise to the RR, but she was late for work and she had to leave before I could try listening to the combination valve area.)
The brake system is the stock C3 booster, C5 master cylinder, stock C3 combo valve, braided hose out to each corner, and Wilwood calipers.

Any productive suggestions or questions would be appreciated.
 
Did a little more investigation, and suspect that the RR wheel bearings (C4 suspension) are going out. I'll pull the thing apart tomorrow and see what's going on. Perhaps it's on schedule, as I replaced the LR bearing last fall.
 
Did a little more investigation, and suspect that the RR wheel bearings (C4 suspension) are going out. I'll pull the thing apart tomorrow and see what's going on. Perhaps it's on schedule, as I replaced the LR bearing last fall.

Yeh, I helped a ZR1 owner changing both his rear bearings....so no surprise....
 
Got the RR wheel bearing changed out (and an inner pad issue resolved which I'll discuss later) and bled the brakes (as I do each spring). With the engine off I get a reasonably firm pedal (it feels like there's no air in the system), but as soon as I start the engine (and have vacuum to the booster) the pedal goes damn near to the floor (it's never been a high firm pedal ever since I put the C5 m/c and Wilwood calipers on it). WTH is happening here? Here's a partial list of what I don't know yet:

1) I transferred the original combination valve to the "new" frame when I put the C4 suspension in several years ago. Is it possible that I could still have some air in it after bleeding the system multiple times over the years?

2) Is the rod between the booster and m/c too short? If I lean my head out of the driver's window and look at the LF caliper area while I'm pumping the brake pedal (during bleeding) I can see fluid (through the transparent hose) come out after only a little bit of pedal movement. I haven't tried this with the engine running, but is there any chance that the point in the booster (that pushes on the rod into the m/c) physically resets itself rearward a touch under vacuum so that the rod length is always too short when the engine is running?

3) If there was a leak issue in the m/c bore/seal area I would expect that the pedal would be mushy all the time, not just when there's vacuum to the booster.

I'm freakin' stumped guys. I got the trailer all hooked up and loaded with tools and stuff, and was backing the car out of the garage to load it into the trailer for a track day tomorrow, and the brakes feel like shit at the moment. Right now I don't feel it makes sense expending the drive time and gas/entry costs to go to the track with little or no brake pedal. Any help would be greatly appreciated.
 
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Mike, I have had my '72 for some 20 years now, driven quickly on the street, not stupid fast, but quickly.....:trumpet: I fought those damn brakes through the lip seals, through a couple master cylinders, brake hoses, brake lines, and even caught that stupid pressure switch up front sucking air into the front caliper lines.....don't ask me HOW, I just know I got one hell of a firmer pedal when putting the rear line to the master cyl rear port direct, putting in a splitter for the front cals, same as rear.....and main line to forward port on the m/cyl......HUGE improvement....but still the pedal soft as a cat......

finally took some advice off CF some ten years+ ago and got a hydroboost, and I even had a huge truck m/cyl in there, trying to get that damn pedal to be firm....well the HB unit stood sharkey on it's nose, and MY nose in the wheel.....put in an aluminum m/cyl of stock diameter pistons.....runs like a charm since.....EXCEPT when the front brakes somehow got cocked over about a years ago, only the INNER pads wearing the new rotors on the INNER surface on the small diameter, not the outer diameter where the pads appeared normal....I have a thread here, where other fellows substantiated the failure as common enough, but no one had a solution....

so new pads and some old rotors I had in the garage from back in never never land.....

BUT, I HAVE PEDAL!!! I hit that thing, and sharkey stands on nose.....:lol::lol:

I know, drove me krazy for many a year, but short trip......OH, ALL other vehicles with vacuum ASSisted brakes, feels like shit now.....3+ inches of pedal to stop a car....really??? GM systems are the worst....5" of pedal.....feels like it anyway.....freeking pedal to the firewall.....;-(


:crutches::devil:
 
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WTH is a brake booster reaction disc?

Took the m/c off again to measure the length of the pushrod and to see if I need to readjust its length. I looked into the booster with a mirror and flashlight, and I saw a shiny surface that apparently is what pushes on the pushrod. However, the cutaway drawing in my Chevy repair manual shows a reasonably thick (rubber or plastic looking) part called a reaction disc that is behind the pushrod. Mine is missing (and probably lost by me when I took the pushrod out to shorten it when I put the C5 m/c on). Anyone know specifically what this disc does, and where I can find a replacement?

Thanks.
 
Well, contacted a couple brake booster rebuilders to try to get a replacement disc, but one won't sell just parts, and the other isn't in a big hurry to answer my e-mail. So, I'm going to see if I can find something in a hardware store that I can cut up and form into a quarter size rubber disc.
Been working on an aluminum adapter to install an '84 booster in the '69. About four pounds lighter than the original booster.

Just for kicks does anyone know the brake pedal movement ratio? Or hopefully I can just look under the dash and slide a tape measure up there to get a rough measurement.
 
I remember taking the OEM booster off my car, back a dozen years ago, but no recollection of any rubber disc in the pedal shaft lineup....all I remember is that the diagram from the overhaul manual says 'valve return spring', just to the right of that 'reaction disc' I have no recollection about....

I blamed the spring for the soft pedal, and excessive travel, too many years now, can't remember more, and of course the booster hit the junk yard and is speaking Japanese these daze.....


:gurney::drink:
 
Well, playing around with the brake booster. I made up a "reaction disc" by cutting up a 1" rubber stopper, and shortening the pushrod a little to make up for the additional length of the disc. The pedal is reasonably firm, but when I start the engine (to get vacuum to the booster) I again get additional pedal travel, and the brakes/calipers lock up, and the pedal stays down until the vacuum bleeds away, and then the caliper releases. WTF?

I'm not bright enough to figure out the issue at the moment.
 
I don't have a good answer but this is a very interesting thread.... hope to hear what the solution is .....

Have you ever tried driving the car and testing the brakes without assist at all ? Disconnect and plug that vacuum line to the booster and see how that works ??? The booster only adds assist, it does nothing for brake performance .... as long as you can apply enough force to the pedal you don't need a booster for the brakes to "perform" ....
EDIT: the booster changes only the pedal "feel" .... but it does not have any affect on the hydraulic side, whatever fluid pressure is required to press the pads against the disc is the same, no booster, hydroboost or vacuum booster...

no idea why adding the vacuum assist would cause additional pedal travel.... its got to be something in the booster... just my 2 cents.... hope I'm not having a brainfart tonight....
 
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I don't have a good answer but this is a very interesting thread.... hope to hear what the solution is .....

Have you ever tried driving the car and testing the brakes without assist at all ? Disconnect and plug that vacuum line to the booster and see how that works ??? The booster only adds assist, it does nothing for brake performance .... as long as you can apply enough force to the pedal you don't need a booster for the brakes to "perform" ....
EDIT: the booster changes only the pedal "feel" .... but it does not have any affect on the hydraulic side, whatever fluid pressure is required to press the pads against the disc is the same, no booster, hydroboost or vacuum booster...

no idea why adding the vacuum assist would cause additional pedal travel.... its got to be something in the booster... just my 2 cents.... hope I'm not having a brainfart tonight....

Years ago I found the same thing, that stupid booster would add inches of pedal travel .....I did all that crap, drop off the vacuum, except for effort the brakes seemed more firm, add the booster vacuum and it was back to grossly excessive travel....the HB setup is not like that....and when tearing the old booster apart, I found a heavy stout spring between the input and output shafts....why there, I dunno....but it would have about 1/2 inch of compression to it, so with that ratio on the brake pedal lever, it would be at least 2" at the pedal end.....:surrender:
 
It sounds like the booster isn't allowing full pedal travel with the engine off. Have you tried bleeding the system with the vacuum lines disconnected?

It also sounds like you either have a missing/defective check valve, something plumbed wrong in the vacuum system, or a defective brake booster. Trying to diagnose these headaches is part of the reason why I went to a Hydroboost.
 
It sounds like the booster isn't allowing full pedal travel with the engine off. Have you tried bleeding the system with the vacuum lines disconnected?

It also sounds like you either have a missing/defective check valve, something plumbed wrong in the vacuum system, or a defective brake booster. Trying to diagnose these headaches is part of the reason why I went to a Hydroboost.

ME TOO Sam, just silly when I tore that booster apart, found nothing rong that I could see, anyway, all the rubber appeared fine, looked at all the mechanical/metal parts fine.......All I can say is bad design, piss poor......I have no where else to go with it.....IF I was the only guy complaining, I say maybe I missed something, fine, but seems like I right....thousands of guys with same issues, except show car guys.....OH, but MY Corvette is OEM, isn't that great!!! with a swish of the hand......:rofl:
 
I guess I'm missing something. Doesn't the pedal drop some and become less hard on all power brake cars? That used to be a test to confirm the booster was working. Bleed all vacuum off by pumping the pedal with engine off. Apply pressure to brake pedal and start the engine. The pedal would drop as vacuum was created from the running engine. If it did not drop there was a problem. I'm confused!

Tom
 
I guess I'm missing something. Doesn't the pedal drop some and become less hard on all power brake cars? That used to be a test to confirm the booster was working. Bleed all vacuum off by pumping the pedal with engine off. Apply pressure to brake pedal and start the engine. The pedal would drop as vacuum was created from the running engine. If it did not drop there was a problem. I'm confused!

Tom

Yeh, that WAS the case with ALL my A & B body GM/Ford/Chrysler cars over the decades.....but for some damn reason this '72 was a totally different bitch/whore.......dunno why, just WAS and I have never read any solid sense making answer as to WHY.....one of life's perpetual mysteries, IF I had driven one of these early sharks when new, I would have a comparison....:clobbered:
 
The connection beetween the master cylinder and the pedal is a rigid steel rod...... so the booster can only act for you likke spinach for Popeye !!!:thumbs:
To increase you strenght.

I would check a couple of things :

Is the car braking with no booster (even with very big pedal effort) ?
Is the pedal having the same travel when reproducing the same stopping power than before but with the booster on?

In any case, if the car will stop even without the booster, the problem can only be elasticity in the system !!!

This can only come from:

1) air in the fluid
2) bad calipers to rotor alignement
3) worn out wheel bearings (allowing rotor to caliper misalignement)


Just my to cents...
 
The connection beetween the master cylinder and the pedal is a rigid steel rod...... so the booster can only act for you likke spinach for Popeye !!!:thumbs:
To increase you strenght.

I would check a couple of things :

Is the car braking with no booster (even with very big pedal effort) ?
Is the pedal having the same travel when reproducing the same stopping power than before but with the booster on?

In any case, if the car will stop even without the booster, the problem can only be elasticity in the system !!!

This can only come from:

1) air in the fluid
2) bad calipers to rotor alignement
3) worn out wheel bearings (allowing rotor to caliper misalignement)


Just my to cents...

Incorrect, there is a very stout spring between the in put and output steel rods, it's about an inch diameter maybe an inch long and made from wire about 1/8" diameter like I say, stiff as hell, look at the diagram.....in the '72 overhaul manual, it's called the valve return spring, Bendix dual diaphragm under brakes, page 5-22......:stirpot:
 
The connection beetween the master cylinder and the pedal is a rigid steel rod...... so the booster can only act for you likke spinach for Popeye !!!:thumbs:
To increase you strenght.

I would check a couple of things :

Is the car braking with no booster (even with very big pedal effort) ?
Is the pedal having the same travel when reproducing the same stopping power than before but with the booster on?

In any case, if the car will stop even without the booster, the problem can only be elasticity in the system !!!

This can only come from:

1) air in the fluid
2) bad calipers to rotor alignement
3) worn out wheel bearings (allowing rotor to caliper misalignement)


Just my to cents...

Incorrect, there is a very stout spring between the in put and output steel rods, it's about an inch diameter maybe an inch long and made from wire about 1/8" diameter like I say, stiff as hell, look at the diagram.....in the '72 overhaul manual, it's called the valve return spring, Bendix dual diaphragm under brakes, page 5-22......:stirpot:

I'm sorry..... but the spring you are talking about is the..... valve return spring !
The free stroke of that spring is in the range of 1mm..... and for your foot is like nothing !!

power_brake_booster_schematic_2_large.jpg


This spring allow a little free stroke of the rod in order to open the vacuun circuit as soon as you release the pedal.
Otherwise.... no way to stop braking !

In other words thi spring is very soon depressed and the Whole system works like a single steel rod.

Furthermore this spring isn't changing the K with o without vacuum !

:nuts:
 
This spring allow a little free stroke of the rod in order to open the vacuun circuit as soon as you release the pedal.
Otherwise.... no way to stop braking !

In other words thi spring is very soon depressed and the Whole system works like a single steel rod.

Furthermore this spring isn't changing the K with o without vacuum !

:nuts:

OK, explain why we all have this same symptom.....that brake pedal drove me krazy for years having some 20 years on this car total, and just with no explanation for that soft pedal, put in HBoost and the difference push my nose into the wheel.....course I had a large diameter truck MCyl on there, so went back to a stock diameter HCyl, aluminum though to cut a pound or ounce off there.....I just figger it to be in the design of the stupid Vac booster, and so a piss poor choice on GM 's choice for production, all I know is my car stops nice NOW....wish the rest of the brakes were not so troublesome too, brake hose left front, new rotors, etc....then to have the stupid brake pads wear on the INSIDE surface and on the INNER diameter too boot on BOTH sided, metal/metal destroyed the new rotors in 4-5 years .....far as I"m concerned it's a shit overall design, and I don't have the $$$ to replace all the calipers....not for the amount of time/miles anymore.....

:clobbered:
 
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