This is one of the best threads on the issue of no brake pedal after replacing brake components, so I'm going to resurrect it and hopefully continue it.Here is my saga:
I have a '68 roadster with standard brakes and had no issue for years. Recently the brake pedal went to the floor and I tried to bleed the brakes as an initial maneuver, but this was unsuccessful.
The brakes were worn and rusty so I decided to use it as an excuse to replace all the brake components. Powerstop rotors and pads. Drivestar calipers x 4. 4 new rubber brake hoses. A Dorman master cylinder.
There were no leaks anywhere in the system that I could detect.
After replacing the calipers, hoses, pads and rotors the pedal still went to the floor despite multiple bleedings with no air bubbles. I ran 2 quarts of fresh DOT 3 through with a Motiv power bleeder. No bubbles except at the very beginning.Brake pedal still went to the floor.
Replaced the Master Cylinder (with the Dorman unit). Bench bled it first per instructions using nylon outlet plugs and a dowel rod. I did notice that the front reservoir (which I believe operates the rear brake circuit) bubbled quite a bit and then shot a mini geyser of brake fluid while bench bleeding, while the rear reservoir didn't seem to do either. I wasn't sure this meant anything and the piston travel hardened up while bench bleeding so I figured it was good.
I put on the master cylinder and also rebled the system again at all 4 wheels (with only a couple of bubbles).Brake pedal still went to the floor.
Power bled x 3 all 4 wheels. No bubbles.
Brake Pedal still went to the floor.
Based on reading this thread, I clamped all 4 rubber brake hoses and tested the pedal. Hard!
To me that meant the replacement Master Cylinder was good (maybe).
I took off the hose clamps one by one and lost pedal when I unclamped the right front hose.
That seemed to indicate the right front caliper probably had air. I ran a pint or so through that circuit using the Motiv power bleeder but no bubbles. Unclamped the brake pedal still went to the floor. Clamped it didn't (though I wouldn't say it was rock hard, but not all the way to the floor).I rebled the right front with another pint while tapping the caliper all over with a dead blow hammer and then a small ball peen hammer to get more vibration.
No air and no change.I thought about it, and then clamped only the left front hose, no others.
Got pedal again!
Tried the same maneuver with each rear hose in turn and pedal goes to the floor.

So putting this all together, I'm suspecting maying the front brake circuit is not moving enough brake fluid when pressing the brake pedal. If that's the case clamping either front brake hose would effectively reduce the brake fluid required to set the pads against the rotor by effectively reducing the overall volume of the front circuit. Not sure yet, but I am considering these possibilities:

1. The Master Cylinder is somehow bad and not moving enough fluid only in the front brake circuit.
2. The proportioning valve could be faulty (the only component I didn't replace other than the hard lines which look good).

I don't know much about the proportioning valve beyond what it is supposed to do, but not sure HOW it does it. Any thoughts would be appreciated.I'll follow up if I get it figured out so as to help someone in the future. My next step is to get my old master cylinder that I removed (not sure if it was good or bad, though no leaks when removed), bench bleed it, see if it hardens up the piston as I bench bleed and also notice whether both the front and rear reservoirs bubble and geyser (recall the new one did this only on the front reservoir). If it passes those tests, I'll put it back on the car and try it out.
If that doesn't do it, I'll probably order a replacement proportioning valve.

 

I would put the old master cylinder back on. Could do that relatively quick and and then a quick pressure bleed will you know pretty dang fast.

 

What is the part number of the Dorman master. Front reservoir is front brakes, rear is for rear brakes. Both reservoirs should squirt.

 

"What is the part number of the Dorman master. Front reservoir is front brakes, rear is for rear brakes. Both reservoirs should squirt."


I read your reply and went and checked the issue of reservoir circuits. I can confirm in my '68 (which is totally original) the front reservoir supplies the rear brakes while the rear reservoir supplies the front circuit. The hard lines cross below the proportioning valve just above the frame.
It surprised me too.

I can confirm that in my case, the problem with the pedal going to the floor is not due to air in the system and it is limited to both front brakes/front brake circuit only. The only thing that can explain this by my thinking is a bad master cylinder or possibly the proportioning valve if it failed in an unusual way.My bet is that it is the new Dorman master cylinder. Part number I bought is M36367. This should be correct (the 1" bore for std brakes).

 

If you still have the original master and had no problems with it try reinstalling it

 

Is your brake light on?

The brake "proportioning" valve in these cars does zero proportioning at all (only on a C2/3), but it does have a "safety" shuttle valve which lights the brake light in the dash when one end of the system loses pressure. It is technically called a combination valve during these years.

There is a tool you should use to bleed your brakes to prevent the shuttle valve from moving.
Once the shuttle valve has shifted it should light up the brake light. But many mechanics used to cut the wire. And many mechanics do not know how to reset the shuttle valve. When the valve is tripped very little or no fluid flows to that end of the car.

I have heard of various ways to reset the shuttle valve, and have tried several. If the wire is cut use an ohmeter on the safety switch to see if it is "on" or grounded. The safety switch thankfully unscrews without introducing any air into the system.


The pressure switch and the shuttle valve are marked. This is a non-corvette diagram which does have a proportioning / brake limiting spring on the rear brakes. The extra metering valve in the front means it is for a disc/drum system. Most C3s have neither of these. But rumor has it is possible the 78+ cars have proportioning, if they do the large round spring at the back would be easily visible, but I have never confirmed it. Maybe someone could take a look or pic. I am unsure why they would have it.

Once you confirm the shuttle is tripped, and the dash light works and is on, open a bleeder on one end, and "jab" the brakes until the light goes out. Or remove the switch and wiggle the valve over with a o-ring pic tool.

 

The brake light goes on only when the pedal goes all the way to the floor and goes off when the pedal is released.
If I clamp either front brake hose the pedal is soft but does not go to the floor and thus the brake light does not come on.
If I clamp both front brake hoses I get a hard pedal.
Clamping one or both rear hoses has no effect - pedal still goes to floor.

I have a second Dorman master cylinder that just arrived and I bench bled it tonight. I'll try to install it tomorrow and see if that was the problem.
If that doesn't fix it, the last thing to do is to replace (or ?reset) the proportioning valve since it is the only thing that has not been replaced.

Fred

 

fwillison,
Comment post 24:

I am 99% sure, someone has tampered with your MC lines to the Prop Valve even if someone claims all original.
As you can see with the Prop Valve photo, it would be easy for someone to accidently or purposely swap the lines around. Look at the "Inlet" ports.
The front rez, to the best of my knowledge, has always been for the front brake system.

 

It's possible, but seems unlikely. It is the formed hard lines that cross below the master cylinder just above the left frame rail, then head forward and back. You would really have to work hard to form the lines wrong and hook them up wrong and it would have happened years ago. It certainly seems weird though. I'll try to post a pic later today.

As far as the proportioning valve pic, that is just a generic photo, it isn't the one on my car.

Fred

 

Yes, I am well aware they are formed steel lines. But who knows if original or from a part supplier.
What threw me was saying the lines crossed. I can't get at my car just yet (storage) to check mine.
But what I am getting at is, maybe the lines were reversed, or wrong port to wrong port.

 

I'd be interested in what you and others find when you follow those hard lines below the prop valve. Who knows? Anything is possible, it is a 56 year old car. But this is a side issue and doesn't directly reflect on my problem. There is no doubt that in my car at least, the lines do cross and the rear reservoir supplies the front brake circuit. Its been this way since I've owned the car since I never touched the hard lines. And the brakes were fine for the past 15 years.

 

I have the second new master cylinder ready to install but now I'm wondering whether I should reverse the crossing hard lines to put the front reservoir in control of the front brake circuit and vice versa.I hate to change 2 things at once and then not know which was the fix.
It would be difficult for the presumably reversed brake lines to be responsible for my problem since it worked fine that way for at least the last 15 years.

Incidentally, I notice when checking Zip Corvette supply that the hard brake lines are unique to a 68. If anyone has a 68 and can confirm the brake line to reservoir routing I would sure appreciate it!
I did take a pic this morning of the brake lines crossing:

 

Look at the brake line diagram
https://www.keenparts.com/CorvettePa...50024&year=C2#

 

Busch lite cans might not be transparent but the beer is!

 

Yep, it looks like mine are not correct. I suppose I'll try to switch the lines before putting in the new Master Cylinder.
Gotta love detective work on these old girls.

 

Here is an internal GM diagram of an early C3 combination valve. The pressure switch in this one is tripped and shifted forward, contacting the electrical switch.



I also found a picture of a 77+ C3 combination switch.
And this one does have the proportioning spring function on the rear brakes!



Here is the GM directions on operation and how to reset the shuttle valve switch.

 

Great info and thank you for the reply.
I have seen that internal diagram on another post and find that it is not exactly the same as the 68 Vette valve. See my markup on the diagram attached. However, the operation should be the same and is very helpful.

 

Well, I did forget that the combination valve is right up top next to the M.C. on early yrs.
Later models, the Prop Valve was moved down and attached to the frame. So maybe the lines do cross. But looking at your photo, its just doesn't look . . . . fack-tree.

It's been said that the fronts do 80% of the braking and the rears just go along for a ride to keep things straight.
So, it just stands to reason that the front rez gets 80% of DOT3.

The name proportioning meant that in a brake system failure, either front or rear, the spring inside the valve will slam to one side thereby "proportioning" the amount of brake fluid to the faulty side. In theory, the master would not run dry, and one could limp home with half your brakes.

Don't forget to bench bleed the new M.C.

 

Based on the great comments and the AIM diagrams, I decided to go ahead and switch the brake lines when replacing the master (again). So now the front reservoir feeds the front brakes and the rear reservoir the rear brakes. Although it worked fine over the years the other way...

I carefully bench bleeded the second new Master cyl. Of course when you remove the plugs and try to get the hard lines screwed in it is impossible not to leak some fluid out of the master cylinder in the process. So I'm concerned that some air gets back in the MC during the install.

I went around and bled the entire system after the line switcheroo and the new MC.

And I still have the same problem, although lesser degree.

The pedal is soft like there is air somewhere, but it does not go to the floor (close but not all the way). And it won't "pump up" working the brake pedal. I actually have enough braking that I took it slowly around the block. It stops but it isn't right, and I could not get it to lock up. And again, I get a hard pedal if I clamp the front brake hoses, with no effect clamping the rear brake hoses.

One interesting point about that is that it seems to rule out a master cylinder problem or a prop valve problem. Since I switched the hard brake lines below the prop valve to the opposite front/rear circuit that was previously plumbed, and the clamping results are still the same - affecting the front only - it has to be distal to the front brake hoses right? Otherwise, the problem would have moved to the rear with the line switch. I think.

Maybe the aftermarket new calipers are faulty. Or maybe there is some phantom air in the front calipers despite pounding on them and pumping at least 2 quarts of brake fluid through them on the Motiv power bleeder. Frustrating and stupefying

 

Well 68 did have a lot of one-year parts!
Hopefully it still has the pressure switch inside.

And for your viewing pleasure here is a NOS GM Adjustable Proportioning Valve # 3878944
Only known factory installation was on L88s, and 4-wheel J56 disc brake Z28s.
Possibly ZR-1s or ZR-2s with J56 HD brakes.
It can be yours for only $2500! LOL On NCRS.com.

And to think I ran one on my 70 Z28 pro-solo car, because it was "cheap" (almost free)!
Who knew?
It is the one mentioned in the Chevy Power Book, which even gave a chart of the pressures at different adjustments. Just turn the ****. It did make a huge difference in how much rear braking power I got. I could finally lock the rears before the front LOL. And when I got it tuned just right, I got up to 1.27G braking. In the 80s.