[konwal455gto]

I have a 65 GTO it had power brakes with stock single master cylinder. The brakes were always touchy , I thought, it was real easy to lock them up. I and my crack mechanic both looked at everything could find no reason why. Moving ahead master cylinder started to leak, brakes became less touchy. Decided to replace with a dual master conversion. New booster and master cylinder for a 68 corvette with drums, was told it was a good for a conversion with stock drums. Now brakes are really , really touchy . WHERE DID I GO WRONG? Thanks Bill

[PMDRACER]

First thing I would check is to make sure the push rod/clevis is in the correct position on the pedal, which would be the lower hole.

Next thing to check would be that there should be about 1/4" of free play after the pedal is all the way up. If not, remove the clevis pin, loosen the jam nut and make the adjustment.

[konwal455gto]

Thanks for the reply, I hope it is that simple Bill

[RamAirIV28]

68 corvette with drums?

I went from a 64 manual to a 66 booster set up. the 64 was a 15/16th dia piston where the 66 was one inch dia. my brakes were really touchy

whatever the original diameter was , you should stay that diameter.

any corvette master cylinder for a disc brake set up will be way too touchy because it will pump too much fluid too quickly to be manageable. even a small increase in diameter produces a large increase in volume.

try a dual master cylinder from a 67 lemans

[chipster]

Return springs worn out and/or shoes contaminated with brake fluid or differential oil from leaking wheel cylinders or axle seals.

[george kujanski]

Mine are real touchy too if the car has been sitting around for a while.......didn't do that when I had manual brakes. I haven't noticed any leakage.

After driving it for a few miles, it seems back to what i would consider normal, still, I'm going to change them, initially go back to a manual master, and then front discs.

george

[konwal455gto]

Happy days, adding 1/4" freeplay calmed the brakes right down. Thanks to all. Bill

[konwal455gto]

I am resurrecting this post because I am still not happy with my touchy brakes. Update I replaced the corvette master with a master for a 67 gto with drums , it has same bore as old single did. No change. I took it to a gm mechanic and he blessed all I had done. His thought is it might be the organic linings. He said to ask the forum if metallic or semi metallic linings would cure the problem because they need to heat up to work. Thanks in advance Bill

[quicksilver97ta]

Just for testing purposes, unbolt the master cyl from the booster, add a washer on each stud to act as a spacer and reattach master cyl. If that fixes the problem then there is an interference issue with rod from the booster to the master cyl.
Also, I have read that old rubber lines can create some really goofy braking symptoms, so unless they are brand new already I would replace those.

[konwal455gto]

Hoses have been replaced , have also tested that the brakes tighten and release at all four corners. Will try the washers. Thanks Bill

[konwal455gto]

I tried the washers as shims on m/c studs no change. Thanks Bill

[Blued and Painted]

Any other symptoms?
Rear brakes locking up early.
Hi or low brake pedal.
Pull right or left.

Is there an alternate hole in the brake pedal for push rod connect

[konwal455gto]

No other symptoms only one hole in pedal gives straight panic stops 4 marks on roads ,no pulling, good pedal. Only what I can describe as touchy. .Not much separates slowing down to full stop. Bill

[fiedlerh]

Could the Primary and Secondary brake shoes be mixed up?

[Goatracer1]

You should check the master cylinder bore. The SMALLER the bore the HIGHER the output pressure. If your new master cylinder has a SMALLER bore than your old master cylinder it will be easier to stop (more touchy).

[konwal455gto]

I checked in two separate parts catalogs and they both say that the orginal single and the 67 dual master have 1.” bores . I inspected all 4 corners and also two mechanic friends took.. : a look everrything is right. We are all thinking that the organic pads may be the problem. So I am looking for some semi metallic pads. So far no luck. Thanks Bill

[chrisp]

Drum brakes require a 10 lb. residual pressure to counteract the spring tension in the drum system which tends to pull the shoes away from the drums. This will give you a longer pedal travel and "spongy" brakes. The residual valve holds a pressure keeping the shoes near the drums giving a higher firmer pedal. Also required a metering valve (PVM) to the front (the metering valve prevents nose dive). A disc/drum combination valve is the easiest way to properly balance your braking system. The combination valve is two valves in one. It provides metering to the front which prevents nose dive and proportioning to the rear which prevents rear wheel lock up. Which are you using ?

[konwal455gto]

I got the conversion kit for dual master from inline tube and installed on new dual master,what they sent is what I installed. Thanks Bill