Thread: New power front disc brakes 67 GTO

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I'm new here. I usually hang out at a Pontiac forum, but I thought I'd try a question here.
I have completed the installation of my front discs, booster, master cylinder, Combo valve and new front lines, and I have a problem. Everything works fine except that after braking the brakes seem to hesitate before releasing. Once I wait a few seconds after remove my foot from the brake pedal, the brakes will release. The pedal is returning back into the up position quickly and fully, so it's not a problem with the brake pedal. I shimmed the master cyl. away from the booster with 2 washers on each side, no change.
Any thoughts on this? Thanks.

Originally Posted by az67goat

I'm new here. I usually hang out at a Pontiac forum, but I thought I'd try a question here.
I have completed the installation of my front discs, booster, master cylinder, Combo valve and new front lines, and I have a problem. Everything works fine except that after braking the brakes seem to hesitate before releasing. Once I wait a few seconds after remove my foot from the brake pedal, the brakes will release. The pedal is returning back into the up position quickly and fully, so it's not a problem with the brake pedal. I shimmed the master cyl. away from the booster with 2 washers on each side, no change.
Any thoughts on this? Thanks.

Can you isolate it to a certain wheel, does one hold and the other release? Could be a caliper hanging up. Another issue could be the combination, are you certain that it is in the centered position????

you could check for free play in the brake push rod there must be free play if not this could hold the brakes and could get bad when heat get in the system from use it would hold the front brakes

Thanks all. I'll check this stuff out. The hoses are new.

Denny, now that you mention it, the calipers are "free" but the rods that run thru the inner caliper to the outer caliper run thru holes that have an o-ring in them and as I recall and it does take a decent amount of pressure to move the calipers in relationship to each other. Not sure how to remedy that.

Dave, I don't know what you mean by the centered position of the combination valve. I just mounted it and connected the lines. The brakes do seem to stop the car properly...

Pat, "you could check for free play in the brake push rod there must be free play if not this could hold the brakes and could get bad when heat get in the system from use it would hold the front brakes"

What do you mean by the brake push rod? Is this the rod between the booster and the MC? If so I thought that the shimming of the MC would help this but it did'nt. How would I check for free play? With the shims in, I do have to press farther to get the brakes to apply.
By the way, nice 66.

Thanks to all. Doug

It could be that the brake cylinder piston is not springing back immediately.

Am I supposed to lubricate the caliper mounting bolts that the calipers glide on?

Once I wait a few seconds after remove my foot from the brake pedal, the brakes will release.

The fluid volume needed to compress the pads is SLOWLY being pulled back into the master cylinder.

The pedal is returning back into the up position quickly and fully

But if you hit it again quickly - before the brakes release - what does it feel like?

I had time tonight to check one thing. I pulled the attachment/glide pins and cleaned then and lubricated the pins. Some of them had some anti squeak stuff on the pins that got in the holes. Cleaning them and lubricating them made a big difference in the way the pins felt sliding back in. After this I took the car out around the block and...no difference.
As soon as I get some help, I'll check the hoses by opening the bleed screw when someone applies/releases the brakes.
Thanks for the help guys.

did you check the brake pushrod for free play???

There seems to be free play for the push rod. I can hold the MC up against the booster mounting area without pushing in the MC cylinder.

i would say look at the master it is for front disc brakes ? sounds like to me it is holding back brake psi . to check push rod just un clip it from the pedal with it unhook you should pull up the pedal up 1/8 or more so that you know you have free play i think the pedal has a stop ? you need to make sure you have some free play and not on the stop if so this will load the push rod

I checked the push rod connection to the pedal. There is at least 1/8" of extra play for the push rod at that connection. I am not keeping the rod from fully extending.
I will get my wife to help me tonight and I'll verify which brakes are not releasing. If it is the front(s), I'll check the hoses by opening the bleed screw.
I guess it could be one of the rears. I did not do any work on them (except to replace the rear brake hose) so I was assuming that they were OK.
The Master Cylinder is new, but I did fully compress the rod into the cylinder when I bench bled it. I later read that this is not a good thing to do, especially on an older unit, but mines new...
Could it just be air in the lines? I need to bleed some more?

I checked the brakes out with my wife working the brakes and here is what I found. The front brakes are not releasing. The rears release fine. If I crack open the bleed screw the front brakes release immediatly. So either both brake hoses are bad, or there is something wrong with the master cylinder. What are the odds that both new hoses are bad? I guess I will replace them first and see what happens. Could the MC be damaged and cause this? Thanks.

Could also be the combination valve hanging up or not correctly centered. Might want to check it, too.