Kris Tunetso

I asked this over in Tech/Performance and didn't get much in the way of answers. Hopefully I'll get more comments here.

This has probably been addressed a thousand times before. I've searched and read several threads here as well as on other forums but cant seem to find what I'm looking for, so hopefully you kind folks can help.

Background:
I bought my 76 in April 2014. It had spongy brakes but it stopped well enough, so I drove it off and on till fall (not the best idea, I know that now). One night I noticed the pedal went almost to the floor before the car stopped, so I parked it for the fall/winter. I finally dug into the brakes back in March and found the left rear caliper was leaking, so I replaced it and put new pads on the car all around. The other calipers appeared fine; nice and dry. I bled the car as best I could, but the pedal never felt very firm. Still pretty spongy. I thought I simply had done an inadequate job bleeding the system, so last Saturday (July 11) I finally decided to pay a shop to do it, just to rule out my own incompetence.

The brakes still feel spongy. They're a little better than before, but the pedal goes halfway to the floor before the brakes even Start to grab. Once I get stopped, there is only about 1/2" to 1" of travel left before it's at the firewall. Now maybe maybe I'm expecting too much, but I think I should have a lot more pedal than that.

I've read that there's a rod in the Booster that pushes the piston in the MC... perhaps that is somehow improperly adjusted? I don't have an Overhaul Manual to check what the proper rod length should be, and my Service Manual doesn't have that information (at least that I can find). Or maybe there is something wrong with the vacuum fitting on the booster?

I've read several times, on this forum and others, that when everything is right the pedal is "rock hard" and the brakes can "throw you through the windshield." Maybe that's an exaggeration, but still, it's better than I have now. I just want good brakes so I can move on to other things. I can't well do much with a car that won't stop reliably. And while it's hotter than Satan's buttcheeks here and my car doesn't have A/C, I'd still rather be driving it than my DD. Got to get this brake problem figured out.

What am I missing here?

74modified

Earlier C3s had an adjustable rod in the power booster (my 74 did) I dont know when that changed. Even without the adjustable rod yours could be short - easy to measure. If it was short, you would feel some slack before the rod hit the master piston. Still sounds like like you need bleeding, if it wasn't a Corvette shop, many regular shops dont know how.

Jud Chapin

My guess is you still have air in the system. These cars can be a bear to bleed. How are you doing the bleeding? I always have done gravity bleeding and never had a problem.

Street Rat

It sounds like you need to re-bleed the brakes in the proper order. The shop probably didn't do that unless they are familiar with older C3s.
Here is a good thread on the sequence...https://www.corvetteforum.com/forums...-proceure.html

Kris Tunetso

Any idea where I can find info for the correct pushrod length before I tear into this? I've not been able to find that info anywhere so far.
I gravity-bleed since I cant get any help. My brother was going to give me a hand but (understandably) his kids take priority and they're always doing something when I need him.
I'm really surprised I have to rebleed the system after having a shop to do it. Now I didn't see what they did, but I guess I just assumed shops have some fancy high-tech machine to pressure-bleed all the calipers at once in order to get all the air out. Am I wrong? Do they only do one caliper at a time? If so, that's the worst $75 I've ever wasted.

DUB

PROFESSIONALLY SPEAKING: DO not concern yourself with this rod adjustment at this time. Actually it is taking PRECISE measurements with what you have currently.

A do-able method for a DIY'er. I have a pressure bleeder that forces the brake fluid through the system under pressure. So it makes it a one-man job.

Hopefully that did sue a pressure bleeder...but I would bet they did not....because the attachment that secures to the top of the master cylinder is unique...and most shops do not invest into this type of tool.

*** NOT PREACHING....BUT...SOMETHING TO CONSIDER*****

If the master cylinder reservoir still has dirty looking fluid in it would leave me wondering how much fluid was used for the bleeding process. DIRTY FLUID and the sludge ( if any) in the bottom of the master cylinder EFFECTS the properties of the brake fluid....thus ...a spongy pedal.

IF when it is BLED OUT...and you get a good pedal that is somewhat UP higher than when you started. And then you fell the pedal is beginning to drop lower. And then when you bleed the brakes again and get air out of ANY of the bleeders....that is an INSTANT indication that a dial indicator needs to be used to verify the rotor run-out....and also check the bearing play.

Old, aged rubber brake hoses can be a partial culprit to a spongy brake pedal. Switching over to the stainless steel braided DOT APPROVED brake hoses will help somewhat....depending if the hoses were a part of the problem initially.

When replacing brake pads on a 1965 to 1982 Corvette...this simple repair concept can have major draw backs due to the design of the calipers. And this does not matter if the calipers are factory lipped seal pistons or the 'O' ring design. Hopefully the calipers are not leaking due to pushing the piston back into the bore and if the bore was contaminated or pitted/worn...the piston seal can leak.

Verifying that the check valve in the booster allows for good air flow is important and that it is a check valve and does not allow air to flow in both directions. A poor air flow (restrictive) check valve can cause for the pedal not to be quite right. Recently repaired one with this problem.

Brake pad material does also effect the brakes when stopping but does not have anything to due with where the pedal begins to stop.

And last but not least...installing and testing the actual pressure at each caliper when the pedal is pressed to see if eh problem is not internally inside the master cylinder...due to possibly being full of sludge....and or if you actually have an internal problem in the brake booster where it will need to be sent out and rebuilt. Boosters can be bad and NOT always show the COMMON PROBLEM of the pedal being really high up and taking everything you have to try to stop the car.

DUB

78SilverCam

I reserved this post from another member in my email; copy/pasted here:

I used to own & operate my own 7 bay repair/machine shop (NJ) so I was able to try a lot of different techniques. I also had a shop in NY State before the 7 bay shop in NJ.

The C2/C3 brake system is not all that complicated relative to other designs.
Once you understand the "physics" of the system, the BS ends.
I actually enjoy doing C3 brakes.... it's so easy.

Bleeding:

First, anyone who tells you the order is important doesn't understand hydraulics.

The 1966 Service Manual has a bleeding order chart in Figure 3 on page 5-2 that specifies RR, LR, RF, LF and on page 5-19 it also says
“If an air pressure bleeder tank is used, operate with a low pressure of 10 to 20 psi to prevent aeration of the fluid”.

It does not address inner and outer caliper half bleeding order.

The 1970 Service Manual says on page 5-4:
“If the master cylinder is equipped with bleeder valves, bleed these valves first, then proceed to the wheel cylinder nearest the master cylinder then, the next nearest and so on until all cylinders have been bleed and there is no evidence of air. The 1970 Service Manual also says on page 5-22: “The rear calipers contain two bleeder valves (one inboard and one outboard) which necessitates the removal the rear wheels for bleeding.”

It also does not address inner and outer caliper half bleeding order.

So, in two GM Service Manuals we have two opposing philosophies… farthest to nearest, and nearest to farthest.

There is no difference in the order in which the calipers are bled... regardless of what they say. I did an experiment to prove this specifically for forum members a while back.
I repeatedly opened up lines on my own C3 and drained them, and re-bled the system in random order. There was absolutely no difference in system performance based on the order of bleed. So don't get hung up on order... it is irrelevant.

I knew the answer to this question before I even started the experiment due to the enormous quantity and variety of brake jobs I did in my shop.

For example, when doing NY State Vehicle inspections, in order to prevent shops from gouging people at inspection time, you are required by law to fix only things that are broken or are a safety issue. So, when a single wheel cylinder (or caliper) was leaking, you had to fix ONLY that one item.... not all four. Although bleeding all 4 is good practice, this meant you had to bleed only that one item if the rest were okay. This is what was done... and the cars brakes all worked fine after the work was complete.

The front and rear circuits are in “parallel”… both are fed simultaneously from the switch. Each has the same opportunity to get air in the fluid, neither is more likely than the other. Taking it to the next level, the two front calipers are also in parallel… both are fed simultaneously from the switch. Each has equal opportunity to get air in the fluid, neither is more likely than the other. Both rear calipers are in parallel running off the rear distribution block… Each has equal opportunity to get air in the fluid, neither is more likely than the other. This is why “order” does not matter.

C2/C3 Corvette calipers are composed of two halves. If you bleed the half FARTHEST away from the hose/line connection first, and then the half CLOSEST to the hose connection, then you can re-introduce air back into the caliper-half farthest from the hose connection. These passages are in “series”. So when bleeding an individual caliper, it is important to bleed the caliper half closest to the hose/line (the inner half) connection first.

Remember this concept instead of left inner right outer right inner left outer right outer blah blah blah etc. It's much easier just to understand the principle as explained above, and you do not have to follow any memorized "order".

Recap…. M/C first, then any caliper, but inner half first.

The M/C (master cylinder) always has to be first because any air in or around the M/C is forced "downstream" where it causes problems.

The first objective is to remove all of the air in the M/C.

To (bench) bleed the M/C, you need fittings & tubes to route the fluid from the exit ports back into the reservoir on top. These used to come with a new/rebuilt M/C, but you can make your own by buying short pieces of brake line and bending it.
Install the tubes & press the M/C piston (pedal) until you get a solid, bubble free flow all around. The M/C is mounted at an angle in the car… air bubbles can get trapped in the highest point of the bore… so try to (un)tilt it to “level” the playing field.
With DOT5, just go nice & slow & steady and you will not generate the tiny bubbles that DOT5 is famous for.

Side Note: The M/C output ports for the brake lines are not drilled in the top of the main bore. They are offset. This means there will be an air space above the ports where air can be trapped. With the M/C mounted in the car, and the car sitting level, this will happen.
Mounted on the bench (bench bleeding), the M/C can be positioned to minimize this problem.

To test the M/C at this point, remove the tubes and replace them with inverted flare plugs, being careful not to allow more than a few drops of fluid to run out of the ports when you switch over to plugs from tubes.

The ports on the M/C are inverted flare, and you can either buy or make inverted flare plugs for them. To make your own, buy brake line with the correct inverted flare fitting, cut them off about 2 inches from the end, use a hammer to flatten the tubing for about 1 inch opposite the fitting end.


Again press the piston (pedal). The M/C should be hard as a rock.
If not, there is still air in the passages, or the internal seals are bad, or in some cases, the piston & seals assembly is put together wrong... I have seen this on new M/C’s.

Once the M/C is finished, it's time to bleed the lines & calipers.

If you have a brake switch (AKA proportion valve), it can cause problems at this point.
Some C2’s did not come with a switch/valve. I believe all C3’s did.

A good, properly functioning proportion valve (misnomer.. it's only a switch) will not allow you to bleed the brakes one line at a time. The principle of operation is such that when one line/caliper leaks (or is open for bleeding), hydraulic pressure (M/C-pedal pressure) will cause the valve's internal piston to slide to one side, activating the dash brake light, and closing off the opposite (front or rear) fluid circuit, rendering that closed off circuit un-bleedable. When this happens, the closed off half of the valve will allow little or no fluid to be bled through that circuit (front or rear).

===> The trick in this case is to re-center the valve and to bleed one front & one rear caliper simultaneously.

Re-centering a poorly functioning valve can be difficult- I have used DRY compressed air forced backwards through the lines to the valve to force the piston back to its center position (dash light goes off), and I have used simple brake pedal pressure to re-center it. The method used depends on how bad the valve is sticking. You may have to rebuild or replace the valve (switch). The air MUST be dry, or you will be blowing water into the system along with the air.

I have disassembled several of these and documented their internal structure, again for the benefit of forum members.

If you can bleed a C3 brake system without dealing with this problem, then the brake switch is sticking and is not functioning properly… which I find on 9 out of 10 Vette’s.

Bleeding the bakes....

Any method that works for you is fine. Everyone has their preference.
The choice of one method over another is a religious experience for some.
“The xyz method is the best and only way to bleed the brakes.” Hogwash.

Key essential point: never let the M/C run dry while bleeding. You must re-bench bleed the M/C and start the line bleeding process all over again if the fluid in the M/C drops to, or below the tiny orifices in reservoir.

Gravity
Pedal
Vacuum
Pressurized

Gravity-
You can bleed a -properly functioning- C3 with a small piece of rubber hose and a coffee cup... no special tools.
All you need is a tool to open/close the bleeders.
The rubber hose is attached to the bleeder, the bleeder is opened, fluid is allowed to flow via gravity until clear & bubble free, and the bleeder is then closed. Don’t expect a fire hose… it runs slow. If you’re in a hurry, then this method is not for you.

To avoid rounding off of the bleeder, use a small 6 point socket to initially crack the bleeders open, but use a box wrench to open/close them while bleeding. The wrench can remain on the bleeder while bleeding, a socket cannot due to your bleeder hose connection. Use a box wrench because it has less of a chance of rounding off the bleeder than an open-end wrench. Tubing wrenches are usually too fat to fit here... good for fittings, but not for bleeders.

You can gravity bleed all or one at a time... place the hose on the bleeder and set it up so that the hose is in the bottom of the coffee cup so no air can travel up the hose backwards once there is some fluid in the bottom of the cup. Open a bleeder(s). Let gravity do its thing. Close the bleeder(s).

Most of the time, fluid will not start to flow all by itself. If not, then a few slow pumps of the M/C with the bleeder(s) open should get it flowing. Remember.... the brake switch can foul things up here. Be aware of its principle of operation and take appropriate action if it gives you grief.

Gravity bleeding is essentially the exact same thing as pressure bleeding, except the pressure is much lower.... only induced by gravitational weight of the fluid.

Lisle (and other companies) sells a small plastic cup with hoses & fittings for brake bleeding for around $10. This is one of the cheapest, effective tools you can use.... similar to a hose & coffee cup. I use these exclusively.

This tool is intended for use with the “pedal” method, not on the "gravity" method.
I use it both ways.


Pedal-

Can be done with two people, or one person & check-valve bleeders.

Two person-
Bottom person opens a bleeder, top person presses pedal, bottom person closes bleeder BEFORE top person’s foot reaches the floor, top person lets pedal back up... repeat.
If the bleeder is closed AFTER the top person’s foot reaches the floor, then air can flow back into the caliper. The two people have to set up an agreed sequence of communication for this to work.

Check valve bleeders come in two varieties. One is the replacement bleeder with the check valve built in. The other is the separate check valve in line with a rubber hose, attached to the caliper bleeder.

Either one works if used properly.
You can buy a set of four and bleed all four calipers at once... but I have found this to be "iffy"… results unpredictable. It's better to do one front & one rear together, and then switch to the other pair. With check valve bleeders, you simply attach a hose/cup to the bleeder and open the bleeder(s) and pump the pedal.

Vacuum-

The "Mity-Vac" tool will work if used properly... but not many people can use it properly. It does have one primary flaw.... it sucks air past the bleeder threads and more importantly, past the internal piston o-ring seals instead of sucking fluid from the caliper. You can minimize this by putting pressure on the bleeder threads while drawing fluid, and by going "slow & easy" with the hand pump. Patience is the key here.


Pressure-

The current "DIY" tool for doing this does not seal properly on the C2/C3 M/C. You have to deal with that issue up front. People use all sorts of rigged up solutions (clamps etc) to make this tool work as promised. Professional pressure bleeders do not have this problem... only the $60 DIY version fails the test. This method is by far the fastest, but does not always allow time for trapped bubbles to escape like the slower gravity/pedal bleeding method does, so even after spending the money and using the tool, you may still have to gravity or pedal bleed to get a firm pedal. Brake fluid is thick (viscous) and trapped air bubbles move slowly... they need a little extra time to migrate to the bleeder orifice. Pressure bleeders tend to move the fluid faster, but the bubbles move at their own pace… they hang up on the caliper casting. I’ve seen this with some of the professional grade pressure bleeding systems with clear site glass… you can see the fluid move while a bubble stays in one spot.

If you use DOT5, then keep the pressure very low or you will introduce air bubbles into the fluid which are an issue with both DOT3 and DOT5.... but more so with DOT5.
One final note on pressure bleeders... since they use non-dry air, they can actually force water (from the air) into the fluid, which will eventually cause internal corrosion. Pressure bleeders should be used with dry air only.


Final thoughts-
In all cases, the angle of the calipers can slow down the bleeding process... air bubbles tend to float up.... not down. They look for a high spot. Just sit back and visualize the bleeder design and location and ask: "If I were a bubble, where would I go?"

Adjust your caliper angle as best you can. Calipers do not have to be mounted to be bled. You can place a piece of wood between the pistons of an un-mounted caliper to bleed it. The wood should keep the pistons all the way in their bores... or... if mounted... use jack stands accordingly to change the angle of the car… even a little bit helps. The key to this is knowing how the calipers are drilled internally... you have to take one apart to see the passages.


Fluid-

A lot of myths on the forum regarding fluid.
If you remove ALL water & water laden air from the system, DOT5 will last indefinitely.
If you disassemble a system at a later date and find water (or rust) in a DOT5 system, then either you did not remove all of the water on the first pass, or your rotors are out of spec and are pumping air & water into the calipers.

DOT5 can't be used for cars with ABS because the pulsing forms bubbles, but is fine for C3's. On my own C2, I converted to DOT5 in 1976. 20 years later (1996), I removed all of the fluid, ran it through a strainer to get chunks of rubber out (deteriorated seals) and then re-used it. There was absolutely no rust or water in the system after 20 years. It does not wear out. It does not melt paint like DOT3/4 does. So you can paint your parts and not worry about the fluid ruining all of your hard work.

Some people claim DOT5 is not good for racing because of its lower boiling point and compressibility... others use it for racing with no problems. It depends on the type and intensity of the racing.

For street, DOT5 has no issues.

DOT3 is cheaper, readily available, has a higher boiling point & is less compressible, but it absorbs water and must be changed regularly or your system will rot from the inside out... including the lines if they're not stainless. DOT3 works fine as long as you follow the rules associated with it... specifically, regular flushing.

Converting from one fluid to another:
Either way... disassemble the ENTIRE system and clean it out manually... do not rely on flushing. This includes the proportioning valve/switch. Flushing is at best 50% effective in removing the other fluid and contaminants... especially in the proportioning valve where fluid does not flow "through" the device. Don't waste your time with flushing... it will bite you later on. A manual disassembly and cleaning is mandatory.

This is just some of the info I have gained over the years... since I work on all cars, not just Corvettes.

Hope it helps.

Tom

74modified

Well you ended up with a lot of info, and all of it looks good. Your push rod would only be off if someone replaced the master with one that has a longer depth. It is easy enough to take the master loose from the booster and measure how far the rod sticks out from the mounting surface, then measure from the master mounting surface into the piston cavity. The rod should only be slightly shorter. But - you sound like you have air or a master that isn't working.
Good Luck

toobroketoretire

You'll have some free play in the pedal arm to the power brake booster but once the free play is taken up you should have a tiny bit more as the master cylinder piston begins moving foreword. With the master cylinder lid off watch the fluid carefully while pushing on the pedal. First you'll have the free play I mentioned then a bit more and you'll see fluid movement in the rear reservoir. The "second" free play I'm referring to assures the piston is retracting far enough to allow the fluid to return to the reservoir.

Jud Chapin

Nice write up!

Street Rat

78SilverCam and DUB,

Thanks for posting some great knowledge and experience!

Kris Tunetso

Yes, thank you for your comments. It's posts like those that make this forum such a useful resource.

I'm probably going to end up taking the master cylinder off and bench-bleeding it. The PO told me he replaced it, and while he had a few other hot-rods he said he "wasn't a corvette guy" so maybe he didn't know what he was doing there. Though, I would think that you'd bench bleed a MC no matter what car it was going on...

Thanks guys.