I put a rebuilt master cylinder on my '69 chevelle last week. It has factory disks front and drums rear. After 1 1/2 large bottles of brake fluid flused through the system, I still could not get all of the air out of the lines. I ended up puting the old master cylender back on and took the rebuilt back to get another one. I put the "new" rebuilt master cylinder on and guess what, I still can't get all of the air out of the lines. Does anyone have any ideas I might try.I have tryed the old way of bleeding with 2 people, and I also bought one of the hand held vacum pumps and tryed what way too. Pedal is still soft. The instructions with the hand held pump said that air can leak around the threads on the bleed valves and make it look like you have air in the lines. So I put the wheels back on and took it for a drive. Still had a spong pedal. Does anyone know what the small cylinder is under the master cylinder? Could it be part of my problem?

Did you bench bleed the MC before installing it?

540 cutlaSS Yes, both times (both cylinders).

The small roundish cylinder is a metering valve which delays fluid flow to the front brakes on initial braking. If it is leaking, it could cause your soft pedal.

It also can complicate bleeding in certain cases. If you were using two people, it shouldn't be a problem, or gravity bleeding works too.

If you are vacuum bleeding or pressure bleeding the front brakes, the valve needs to be held open. Below and above certain pressures it will not allow fluid (or air) flow to the front calipers. There is (or was) a rubber boot on the firewall side which has a button under it. This button needs to be either held in or pulled out when pressure or vacuum bleeding.

Other possibilities can include, but are not exclusive to
No residual valve inside new master may be allowing air to be sucked in past the rear wheel cylinder seals every time you let off the pedal.
or
Possible mismatch of pushrod to master can complicate bleeding and cause a low pedal.

I would make sure the pushrod matches up, and then gravity bleed it for a while, then test the pedal. Try to determine if it is okay on the first hit, and gets lousy if you pump it. And try to determine if it is the front or rear system with air.

Is the pedal better with the old master cylinder?

Chevry, can you explain how to gravity bleed the lines. Thanks for your help.

Take the cap loose from the master, and open the bleeders at the wheels. Let the fluid drip and dont let the master run dry. Gravity does the rest.

Sometimes it doesn't work on drum brakes because flow is restricted by the residual valve, but it works for front discs, and can even be found as recommended procedure in some old dealer books.

I usually let it drip until it seems to flow freely, close them up, hit the pedal a couple times, and open a bleeder looking for air. It should be gone.

Chevry, thanks for the help, I'am gravity bleeding the lines now.

Good news, the gravity bleeding worked. The soft pedal is gone and the brakes work fair. The problem now is that I can't get the brakes to lock-up, no mater how much presure I put on the pedal. The new motor and cam only has 9 to 11 units of vacum at idle. How can I test the power booster to see if it is ok? If the vacum booster is ok, how do you determan if a vacum can is needed? Again, thanks to you all for the help.

There should be some decent instructions on testing the booster and its checkvalve in your service manual. If you dont have one, you should get one. A Chilton's will do.

You are probably correct in that you need a canister due to the thumpy cam. 9-11"HG sounds low to me.

Glad to hear the gravity bleeding helped with the soft pedal.

On the gravity bleed, doyou open all bleeders at once? Or do you do rear right, then rear left, front right, front left?
I still have some air in my lines too.

1 at a time. do pass rear, driver rear, pass front, driver front. In that order.

For gravity bleeding, I usually open them all up at once. If one seems to be flowing well, and another isn't, I'll close the one that is flowing. No flow can sometimes indicate a restriction...