kellyr073

Troubleshooting my friends 66 after he did an upgrade from a single line to dual line master. Since replacing most of the system to make it "safer", he had had a dissapearing break pedal, only after driving on the highway for 15-20 minutes. Around town after bleeding no issues. Even was in a parade this weekend for at least an hour, no issue.

Pedal is hard like it should be after bleeding. Do a test drive around town, 15 minutes or so, feels great.

Take it on the highway, and after 15 minutes or 10 to 15 miles, the pedal goes to the floor and never comes back until we bring it home and bleed the brake lines.

First time we bled them, I got foam out of the passenger rear. Cleared up after a thorough bleeding. Second time I got foam out of the drivers front.

Replaced master cylinder (twice)
New stainless break lines
Rebuilt orginal calipers
Rebuilt trailing arms
Cut orginal rotors

I belive the proportiion valve was new since he went for single to dual lines.

Manual brakes, no booster.

One persons mentioned that the tolerance or runout from the trailing arm could cause the calipers to oscillate inducing air I to the system cause the loss of brakes. By if that's the case, should t the proportioni valve prevent total loss? Any help is appreciated. Thank you.

corvettekent

You should put a dial indicator on the rotors. It sounds like you are pumping air into the brake system.

JohnZ

Changing the master cylinder is unrelated to the symptom you have (classic/chronic rear caliper air-pumping due to excessive lateral runout). What other work was done on the car along with the brake work? Whatever other work was done is the cause of your air-pumping problem, or you would have had the problem before doing any other work.

DUB

I agree with corvettekent and JohnZ.

It is a MUST that you measure your bearing run-out...and then the rotor run-out.

DUB

Bluestripe67

Then when you mate them, index the hub and rotor with a permanent mark. Dennis

kellyr073

Thanks for the quick feedback. Sounds like everyone is on the same page with the runout being the culprit. Makes sense. Thanks, and I will follow up after we get that checked out. Good tip on the stamping too! Thanks.

John S 1961

can you pump the pedal and get a pedal back?, sounds like bad pad knockback

DUB

This idea is GOOD for only the front rotors. It can not be done in the rear rotors. They can only go on the spindle ONE WAY. UNLESS you do not care about having to adjust your parking shoes.

Also....make sure that the surface of the spindle or front hub is clean and free of debris...and also the inside of the rotor where they both touch. TRUST ME...a little bit of crap can throw off your measurement DRASTICALLY. Make sure you carefully torque the lug nuts CORRECTLY so it pulls the rotor down evenly and correctly.

DUB

kellyr073

1. The right rear rotor was warped.
2. Drivers hub was bent. Rotors were getting thin.

A.Recut rear rotors and reinstalled.

B. Got a "new" hub, installed 2 new front rotors, and retracted with screws. Run-out almost zero.

Re-bled all brakes, now need to take it for a long ride.

Will update shortly, but the theory is proving itself true so far. Thank you.

irwiny

I was researching the same conversion for myself and I noticed that list of items changed didn't include both front and rear distribution blocks. This is needed since your converting a single to a dual master cylinder.

I was told the new line to the rear is normally sold as two pieces, if our not doing a frame off type upgrade. Maybe there is an issue at the junction on this line?

Here is web link convert single to dual master cylinder:

http://www.parts123.com/corvettecent...50B&ukey=33693

Below link is for same w/booster, check the tab Kit Includes for the complete list (excluding booster).

http://www.ecklerscorvette.com/corve...1963-1966.html

Good luck and PM me on your upgrade status, I'm planning to do the upgrade before summer.

Irwin

kellyr073

Irwin,

We did the upgrade to the dual line master, along with new TA's and a full rebuild of the suspension. Btw, we used a 1peice line for the rear, with the body on. Did the same on my 71 and another 70 recently. No issues to note there, pretty straight forward if you're careful not to crimp the line while massaging it into its home.

irwiny

kellyr073

Any reason why you didn't add the booster and did you use a dual master cylinder like 67 version?

Seeing that my car is a resto-mod, I can't see myself spending $700-900 for a dual that looks like the original.

For me this upgrade is one of the things to do on my list of other items.

My brake pads still look good, the major master cylinder / caliper brake job upgrade/rebuild will be maybe later this year if the weather becomes more cold. So the this upgrade is a nice to have done along with some other small issues that pill up and slowly goes down.

Thanks.. Irwin