That seems most likely.

Side note...

No matter how thoroughly you flush the system, and regardless of what you flush it with, there is no substitute for a mechanical disassembly and cleaning.

This is especially true in regard to the brake switch/valve.

Either disassemble & clean it, or replace it. It doesn't "flush".

I have run a series of documented tests on these (here on the forum) and I have disassembled and photographed the internal parts. You cannot flush them.

The master cylinder does not flush well either.

Voice of experience.... I was in the repair business and did a lot of these.

 

brake switch/valve? Are you talking about the Proportioning valve, if so I'll just replace it.

How would I go about using my air compressor to flush the system? How much brake fluid would I need? Thanks guys, You've probably saved me a little bit of money that can go into suspension parts.

Josh

 

Yes... which year C3 do you have? (Duh....75)

"Proportioning Valve" may be a misnomer.... it's just a switch... a hydraulic valve slides to one side when a leak occurs to stop all of the fluid from instantly pouring out. I tested these to see if they really work.

The proportioning on a C3 is done by the relative sizes of the front and rear caliper pistons... front pistons are bigger.

If you use compressed air, make sure it is thoroughly "dried". Compressed air right out of the tank is loaded with water.
If you do not have a good drier, you introduce moisture (AKA: death) to the C3 brake system.

You only need to run fluid through it until it comes out clear at all 4 calipers.

Pick up two quarts and you should have more than enough.

 

I'm not good at bleeding brakes and tend to shy away from it. So, let me get this straight. To flush I need to open all bleeder valves or flush system when I replace the Flex hoses? Just pour the brake fluid in the MC and press brake? Sorry I seem ignorant on this, but I am(very). This is one of the few things I take my car to a mechanic for.

 

Here is something I wrote a while back... I have it in a WORD doc because the question pops up about every 3 weeks.

**************************************** *************

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.

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

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.

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. 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 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.

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 not allow any 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.
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

 

Replace the flex hoses, then bleed.

 

Thanks a lot. Looks like I will get some check valves as well. That has been my problem is communicating with the other person. I'll have my hands full next weekend.

 

I not trying to undermine anyone's life experiences here but my GM '72 shop service manual states:

"Sequence For Bleeding Wheel Cylinders


It is advisable to bleed one wheel cylinder at a time to avoid allowing fluid level in reservoir(M/C) to become dangerously low. The correct sequence of bleeding is to bleed wheel cylinder, either front or rear system, nearest master cylinder first. This sequence expels air from lines and wheel cylinders nearest to the master first and eliminates the possibility that air in a line close to the master cylinder may enter a line father away after it has been bled."

Surely following this sequence can't hurt the process and might add a measure of safety to the result. Since we're talking brakes here I thought nit picking might be wise.

 

Yup... it's a hard habit to break... I will bleed them in that order just out of habit... "old school".

If you draw a simplified diagram of the system, unless air has some form of intelligence, it cannot differentiate whether it is in a long line or a short line.... so it makes no difference.

That's why I did the test... to see if air was smarter than me.

 

I ordered some stainless steel brake lines and some speed bleeders from summit. Since it's 10.95 Handling per order I went ahead and got a Flexlite fan and spacer. I'll bleed the brakes after new hoses and hopefully, On to the next system!

 

Shesh, I'd better not run that test then!

seriously, Nice writeup about bleeding Tom!

 

The fact that the gasket on the master is swollen concerns me. I think you may have been contaminated!!!

 

Good gawd we wouldn't want to be perceived as being that way would we...

So what you're saying is every car that's on the road or ever was on the road has no bleeding sequence and we can either change our foolish ways or forever be thought of as a babbling old fart?

 

The flex lines can collapse internally and look ok on the outside. They can hold vacuum and lock the brake, and then release the brakes as it goes away.

 

I was always taught to bleed like this....as some have said, calipers the most close first. The difference is we have bleeder closed, pump brakes several times and crack open bleeder while still having foot on pedal and close as pedal gets to floor.

 

Yeah, that's what i was thinking. I will flush with as much fluid as possible next weekend, when my parts come in.

josh

 

Josh and others,
Many years ago I had a neighbour who had his VW (Golf or Passat, can't remember) serviced before he went for a holiday up in the mountains. Car performed perfectly but on his way home, it seemed to lose power. He put his foot down further but after a while the car kept slowing down. Then he noticed the smoke and the smell of burning fluid. He pulled over and the car "stopped on a dime" with smoke pouring from the front wheels.

Even when it had cooled down, the car stalled when he tried to take off. He had to get it trucked home. Turned out that the mechanic had topped up the master cylinder with engine oil and after several hundred miles, it had worked its way through the braking system and had caused the rubber seals in the wheel cylinders/calipers to swell and not let the pistons retract when pressure was released.

He stood his ground and the workshop did a complete overhaul of his brakes AND put in the RIGHT fluid. At their cost.

Maybe Josh, a PO had accidentally put some mineral oil in your MC and that is causing the rear flexible lines to swell and "lock the brakes on." More than likely not, but you never know.

Hope you have fun with your new toy.

Regards from Down Under

aussiejohn

 

Yeah I have heard of some pretty strange things being mixed with brake fluid...ATF being just one example!!!

 

Wise man to never say never.

 

Went to replacing my brake hoses on Sunday and found both rear hoses(only ones I've seen so far) had a Big bubble on them. The guys who mentioned the hoses giving out were right on. The only problem is I found out my rotors, pads, Calipers, and all brake lines need replacing. Good and bad. Good b/c I'm glad I found out now and not after everything was bolted back up or I was on the road. Bad b/c I have to spend some more money that I was going to save for the complete suspension packageas(FandR). I also got my rear shocks installed, look pretty sweet. Haven't been able to test them out due to the fact that I have no brakes now. Thanks for the help and being right on point guys.

Josh