'74 brake pedal failure

[Mel Goff]

I just sold my '74 big block coupe. The new owner was driving the car home (about an hour duration)and experienced loss of brake pedal, to the floor....he was able to 'limp' the car home, thank god.

I had replaced the master cylinder a couple days ago with a Delco unit, we renewed all system fluid with fresh, unopened DOT3 and bled the system. I took the car out for a road test and the pedal was firm with no problems and had excellent braking.

This car was painstakingly restored to factory new condition, all wheel cylinders replaced with SS calipers, all brake lines and components were replaced with new.

I had this same episode happen to me last year, that is why we decided to replace the master cylinder. To further compound the puzzle, there are absolutely NO external brake fluid leaks anywhere on the car, everything is dry as it should be.

It should be noted, I didn't have a vise available so we did NOT bench bleed the master before install, but after a thorough bleeding at each wheel, there was no air exhibited in the fluid. What culprit could be causing this?

Thank you.....Mel

[Joe L.]

Re: '74 brake pedal failure

Mel------

There are several possibilities:

1) air in the system due to failure to bench bleed the master cylinder is right at the top of the list;

2) air in the system due to failure to completely evacuate it during the bleeding process;

3) "air pumping" phenomenon due to excessive runout on the rotors (FRONT or REAR). If the rotors have ever been replaced on this car or if the rotors have ever been removed from the spindles (rear) or hubs (front), then air pumping is a strongly possible culprit. Check the rotors for removal of the rivets. If so, measure the runout on the rotors as described in your Chevrolet Service Manual. Any runout exceeding .005" WILL result in air pumping. Air pumping is usually manifested by obtaining a satisfactory pedal after bleeding and then loss of pedal after driving the car for awhile. The length of time before the pedal problem occurs is usually proportional to the amount of excessive runout; the greater the runout, the quicker that the pedal loss is experienced.

[Jim T.]

Re: '74 brake pedal failure

Agree with everything Joe has said. Once upon a time my 70 had this problem after a big time Chev dealership in Dallas did some warranty work on rear wheel bearings. They removed the rivets to service a wheel bearing problem. I could jack up the rear wheels and remove them, put a few lug nuts back on to tighten the disc to the spindle and put the car in gear and watch the pistons move in and out because there was so much runout. I could also go down the highway and lose brakes on the rear because of the pumping of air. I would bleed the rear calipers and get the air out and have good pedal again. Go down the highway get air in the rear calipers again. Problem solved when I sent the spindles and disc rotors to Bairs and had the rivets installed and trued as an assembled unit. If only the information that is obtained from this discussion board was available in 1973 it would of been great.

[Joe L.]

Re: '74 brake pedal failure

Jim-----

A lot of the information disseminated on this discussion board was being LEARNED in 1973. So, folks today with problems get the benefit of "those who have gone before". That's the beauty of this discussion board.

[Mel Goff]

Re: '74 brake pedal failure

Joe and Jim.....

Thanks for your input guys....as always, your knowledge and willingness to help others is what keeps this forum on top.

Thank you,

Mel Goff