67 C2 master cylinder with bleeders

[Gerry G.]

Still have poor pedal and poor stopping.Every thing new.I bled today useing motive bleeder.First I bled the hole system and got no air.I ALSO BLED THE BLEEDERS on the master.Before I disconnected the motive bleeder I checked the pedal an it was like a rock .Best pedal ever. After I took unit of car checed pedal again an it was soft and it did not stop well on roade test.Prior to all of above ITried a perculation test on both resivoirs an the rear resivoir faled. Could not see any move ment at all.thinking about getting a rock auto m/ cylinder.What does any body think???

[Joe L.]

Re: 67 C2 master cylinder with bleeders

Still have poor pedal and poor stopping.Every thing new.I bled today useing motive bleeder.First I bled the hole system and got no air.I ALSO BLED THE BLEEDERS on the master.Before I disconnected the motive bleeder I checked the pedal an it was like a rock .Best pedal ever. After I took unit of car checed pedal again an it was soft and it did not stop well on roade test.Prior to all of above ITried a perculation test on both resivoirs an the rear resivoir faled. Could not see any move ment at all.thinking about getting a rock auto m/ cylinder.What does any body think???

Gerry------

I don't think that bleeding the master cylinder at the master cylinder bleeders works very well. You might try bench bleeding the master cylinder. I think this is probably why GM eliminated the master cylinder bleeders about 1973.

Otherwise, all I can say is this: brake bleeding of any 1965-82 Corvette is the job I fear most of any other. With my other cars I've never had a bit of trouble but with a Corvette endless frustration.

[Ray G.]

Re: 67 C2 master cylinder with bleeders

Hello;
"Everything new" master, lines, and calipers were all dry.
My suggestion is to either bench bleed the master or have someone help with the bleeding by pushing the peddle.
Make sure you have free play before the actuating rod starts moving the piston in the master.
If you have a shop manual, follow the procedure in the manual.
We gravity bleed the calipers by starting the fluid flow with a hand operated vacuum pump. Start with one front & one rear caliper, using the
inner bleeder first. When you have fluid flow at the caliper stop using the vacuum pump.
Pressure bleeders sometimes force bubbles in the fluid, especially if you DOT 5.
My .02

[Danny P.]

Re: 67 C2 master cylinder with bleeders

Bench bleed the Master first, then open bleeders front and back and let it gravity bleed until no air bubble show , keep refilling master fluid at the same time.

[Joe R.]

Re: 67 C2 master cylinder with bleeders

Still have poor pedal and poor stopping.Every thing new.I bled today useing motive bleeder.First I bled the hole system and got no air.I ALSO BLED THE BLEEDERS on the master.Before I disconnected the motive bleeder I checked the pedal an it was like a rock .Best pedal ever. After I took unit of car checed pedal again an it was soft and it did not stop well on roade test.Prior to all of above ITried a perculation test on both resivoirs an the rear resivoir faled. Could not see any move ment at all.thinking about getting a rock auto m/ cylinder.What does any body think???

I have done dozens of brake jobs on cars other than Corvettes and never had a problem bleeding the brakes. However, on my '67 after a complete rebuild (all new lines, rebuilt calipers, and rebuilt MC), I nearly went insane trying to get good pedal. I tried gravity bleed, vacuum bleed, and pressure bleed to no avail. Eventually I removed the MC and did a bench bleed on it, and that fixed the problem.

Here is my theory. I think that simply pushing the brake pedal to the floor does not push the MC piston in 100% of the travel, and some air remains in the MC. This problem is worse if you have power brakes, which tilt the MC at a steeper angle.

I do not think it is mandatory to bench bleed the MC, but if the MC is installed on the car, the following steps should be taken:

1) Put the front of the car up as high as possible to force air in the MC all the way to the end of the bore.

2) Remove the pushrod connected to the brake pedal and have an assistant forcefully push the MC piston all the way to the end of its travel as you bleed the MC.

3) When you re-install the MC pushrod, make sure that you adjust the pushrod to leave some free play when the pedal is in its fully-off at-rest position. The MC piston has to fully return to the fully-off position for the internal self-bleeding feature to properly operate.

For some reason, the '67 system is very difficult to bleed. So, don't feel bad if it's giving you trouble. Many of us have experienced similar problems.

[Joe L.]

[Bill L.]

Re: 67 C2 master cylinder with bleeders

Gerry...(and Joe)...

Bench bleeding is the way to go.... I changed out an after market master on my 67 coupe and my mechanic did a bench bleed....

Perfect off the bench.... highly recommend giving that a try...

Regards
Bill

[Mark F.]

Re: 67 C2 master cylinder with bleeders

How did they do it on the assembly line?

[Terry M.]

Re: 67 C2 master cylinder with bleeders

How did they do it on the assembly line?

They had a fixture that attached to the master cylinder in place of the cap. It pulled a vacuum, held it at a specific level for a specific time to check for leaks, then introduced a specific amount of brake fluid into the master cylinder. Since the entire system was under vacuum (or negative pressure if you prefer) fluid flowed to all of the system.

FWIW: The same process was used for the coolant, and is used today and on other makes and models of vehicles.

[Joe L.]

Re: 67 C2 master cylinder with bleeders

They had a fixture that attached to the master cylinder in place of the cap. It pulled a vacuum, held it at a specific level for a specific time to check for leaks, then introduced a specific amount of brake fluid into the master cylinder. Since the entire system was under vacuum (or negative pressure if you prefer) fluid flowed to all of the system.

FWIW: The same process was used for the coolant, and is used today and on other makes and models of vehicles.

Terry------


I thought that John Hinckley had once said that they introduced fluid under pressure at the master cylinder and pulled a vacuum at the wheel cylinders/calipers?

[John D.]

Re: 67 C2 master cylinder with bleeders

It would be interesting to know when GM started evacuate and fill for brake systems in the plants. Years ago when I worked in an assembly plant on brake fill equipment one of the old timers told me about how they used to purge the brake systems on line with a pressurized fill head and then open each bleeder with a wrench that had a vacuum hose attachment with a clear line. They would purge until the line ran clear and the fluid was recycled back to the fill equipment.

One thing I remember is the system for delivering the brake fluid to the fill machines was complicated as well. The huge stainless brake fluid totes were warmed and the fluid was "conditioned" both by warming it up and storing it under vacuum to boil off any contaminants.

[Mark F.]

Re: 67 C2 master cylinder with bleeders

...One thing I remember is the system for delivering the brake fluid to the fill machines was complicated as well. The huge stainless brake fluid totes were warmed and the fluid was "conditioned" both by warming it up and storing it under vacuum to boil off any contaminants.

Hi John, I'll bet that also helped prevent moisture from getting into the fluid, too...water and brake fluid are not a good combination...

[Terry M.]

Re: 67 C2 master cylinder with bleeders

Terry------


I thought that John Hinckley had once said that they introduced fluid under pressure at the master cylinder and pulled a vacuum at the wheel cylinders/calipers?

What I described is what I understood George Barlos & Phil Hawakens to say. Could the system have changed over the years? My focus was on chrome bumper C3s, although I can't think of why C2s and any C3s would be any different from each other.

John Daley's mention of warming the totes makes a lot of sense, even though I don't recall either of those men mentioning that.

The system I described is also the same system I watched them use on my C6. I don't recall seeing anyone at the brake calipers. There was a tote sitting beside the line and It wasn't warm. I got close enough to see that it was DOT 3 that they filled the brake system with. DOT 4 for the clutch. But then we weren't talking about C6s above.

[Gerry G.]

Re: 67 C2 master cylinder with bleeders

Still having an issue with gm licenced master cylinder.I cannot get a good pedal.cssb sent me a new one and still have the problem.I took the master apart and found no defect; Except on the stop screw,it does not screw in to the bore at all,to short. Can any body tell me if this is a problem? I have taken masters apart that did not have this screw however had a threaded hole for one.the screw being to short can not perform its function as a piston stop.

[Joe L.]

Re: 67 C2 master cylinder with bleeders

Still having an issue with gm licenced master cylinder.I cannot get a good pedal.cssb sent me a new one and still have the problem.I took the master apart and found no defect; Except on the stop screw,it does not screw in to the bore at all,to short. Can any body tell me if this is a problem? I have taken masters apart that did not have this screw however had a threaded hole for one.the screw being to short can not perform its function as a piston stop.

Gerry-----


The lack of the piston stop screw has absolutely nothing to do with your problem. Many commercial rebuilders will delete this screw in rebuilt master cylinder originally having the screw. I disagree with the deletion of the screw but it does not seem to cause a problem and certainly not the kind of problem you're having with bleeding the system. All 1965-77 Corvette dual master cylinders had the screw. However, GM deleted it after 1977 although the master cylinder design remained the same as 1968-77.

I don't know why the reproduction master cylinders have a screw which is too short to be effective. It may be that they only install the screw for "appearance" purposes and not functionality having deemed the functionality not necessary.