GATOR2511

I'm an old guy so I was raised on drum brakes. MY '64' vette has a brake problem that is driving me bonkers!! I have replaced the brake system complete with the exception of the steel lines. The first half of the pedal is spongy and the next half will lock up the right front and slam you into the windshield. I was told by a resto shop that it could be my proportioning valve but I can't find it on the car or in the build manual. I don't think it came with one cause the brake lines are exactly like the manual. Any ideas to lift me out of the confusion? :crazy: :crazy: :confused: :cry :cry :cry :mad

Top_Notch

There is none to my knowledge. I believe they just have what they
call 'distribution blocks'.

Chuck Gongloff

No proportioning valves on 63 or 64 drum brake cars. A couple of suggestions....first I would check all the wheel cylinders. Very often, you will find a mix and match set of different "bores" on the fronts and rears. Don't remember what the correct bore for each is right now, but they should be the same...both fronts equal....both rears equal. Check the bore on the master cylinder and see if it's the "original". Very often these are replaced by "look alikes" without the correct piston diameter. Finally, check the steel lines and rubber hoses for obstructions, AND check to see if your brake shoes are properly installed and working smoothly when you depress the pedal. Also....??? drums out of round?? Hope this helps. Chuck

GATOR2511

Never heard of the bore difference theory :eek: but thats a good lead. The wheel cylinders are new but "new isn't always new anymore". All the rubber hoses are new and I marked the shoes with a magic marker and checked the wear pattern. Perfect! I also had the drums turned and had to replace one on the rear. I've had three master cylinders on the car with the same results. :confused: The car stops good unless you really stop hard then the right front locks up. Anybody want to buy a '64' with a brake problem? :crazy:

MasterDave

My brakes are all new including the master cyl. and although it does not slam me to the windshield, it does pull (yank the steering wheel) ocassionally when braking. Sometimes left sometimes right. I've used the adjusting tool to make sure both brake shoes have the same gap btwn the shoes and the drum surfaces and that helped, but did not take it away completely. I've heard the single brake master cyl is part of the problem, but do not know for sure. Hope you find the problem and let me know what it was.... :confused:

kellsdad

I suspect one of the true experts will come along with a solution to your problem. But in the mean time, I'll offer comments from an amateur with a 63 Vette. The single brake master cylinder sends brake fluid under pressure to the brake block near the left front wheel and from there, to all four wheels. Unless there is an obstruction in one of the brake lines, the fluid pressure going to all four wheels is the same. So, if one of the wheels is acting differently from the rest, there must be something different about the brake assembly on that wheel. It's probably impossible to adjust the brakes so all four wheels lock at the same time, so it is reasonable to expect one wheel to lock up before the others. But it isn't normal to have the steering wheel pull strongly to one side under heavy braking.

GATOR2511

I think you are exactly right! I tried adjusting the right front shoes so they were really loose. Logic tells me that in that case some other wheel will lock up first cause the right front is looser than any other wheel. BUUUUT, the right front locks up first anyway. I changed the return springs on the shoes today but to no avail. :(

plaidside

I am doing a 64 brake system as we speak.
First, the front bores are 1 3/16", the rears are 1", and the master is 7/8".
As to the pull on the right front, I also suspect something wrong in that wheel or the left brake is not working, causing the right one to pull. I would make sure the both front wheel cylinders are working. Remove one drum at a time and have someone slowly depress the pedal, while using two screw drivers to hold back the shoes, see if both plungers are working. If you have a collapsed brake hose or metal line to the left side, this would reduce the pressure to that wheel, causing a pull to the right.
Also check the front end, if one side has a worn component, it can cause a nasty pull.

Double check the master, most masters of this style are 1" bore, the look the same, but our Vettes uses 7/8" bore.

GATOR2511

Thanks for your help guys. Now I have more to go on. The people on this forum are great!! :cheers: :cheers:

GregP

Since it's soft, soft, ON! ... I'd look for a burr or something that is keeping the rear shoe from moving and "energizing" the brake system smoothly.

Would having the backing plates on backwards (right for left, or two left) make the "self-energizing" not work correctly?

-Greg

64BB

GATOR, when i got my 64 BB, it had been bubbabed but good. my brakes did what sounds exactly like what yours are doing. they were a little spongy, and then out of the blue, with normal pedal pressure, they would massively grab, pull sharply to the LEFT, and leave a skid mark [passengers towards the windshield]. i was afraid to drive the car because these wierd "emergency " stops were going to get me rear ended. i tried the obvious things. i bled the system; adjusted the shoes; honed out the cylinders and used rebuild kits; looked for burrs and grease everywhere. still no luck. FINALLY, I NOTICED THAT AN INNOCENT PART WAS MISSING FROM THE LEFT FRONT. there is a diamond shaped piece of metal that fits between the top of the brake shoes and seems to be used to make sure the tops stay on the pivot. without this, it is possible for the shoe tops to move away from the center of the car and for the shoes to get too close together at the tops. i found a used piece for $3 and slipped it in. three years later, the problem is still cured, and the result was instant and dramatic. just my two cents worth. good luck. :smash:

andy60

Gator,

The spongy pedel is the key that some part is not working the way it should and I expect the wheel cyls or emergency cables are hanging up.

No valve on that year. Are you sure the drums are smooth and true? I need to agree with GREGP on this, are you sure the movement of the rear shoes are not getting held up some how.
On a 63 I had the parking break cables were so rusty they were preventing the shoes from moving until the wheel cyl built up enough pressure then "SLAM" they would activate. There are lot's of things that could be preventing your shoes from operating. To check this you need to jack up the wheel and move the drum out an inch or so and hold it out while someone works the brakes. Do not try to work the brakes with the drum off or your wheel cyl pistons will pop all of the way out (at least they should if they are working well).
I hope this helps.. Andy

SWCDuke

On Corvette disk brake systems, the bias is very simple to compute. It is just the ratio of the front and rear piston area to the total piston area, because the Corvette's front and rear disk and pads are essentially the same. I think the ratio computes out to about 65/35.

On drum systems it's a bit more complicated because the front and rear drums/shoes are different, so the total bias is a function of both hydraulic bias and the relative torques developed by the front and rear brake hardware at the same line pressure. I recall that the basic hydraulic bias is 58/42.

The overall bias was set up based on the available grip from the OEM bias ply tires. A modern sticky tire might cause rear brake lockup because their increased grip will increase deceleration, which will increase front weight transfer. The cure would be to go to a smaller diameter rear wheel cylinder. It's not possible to analytically determine the size, so you'd have to just try smaller cylinders in 1/16" increments until you find one that normally locks the fronts first, but not by much, and you'd have to come up with a compromise for all conditions from wet to the stickiest dry pavement.

Another issue that can upset brake bias is a dragging rear parking brake. If in doubt remove the cables and clean them out with a good solvent, then lubricate them with a graphite-based lubricant like Lock-Eze. Do not lubricate them with any kind of petroleum-based product.

The only Corvettes that have an OEM proportioning valves are the J-56 equipped examples beginning in 1966. These valves were adjustable so the brakes could be tuned to the available grip. Some say the later J-56 system did not have an adjustable proportioning valve, but I have never been able to confirm this contention.

Duke

smithd7

GATOR,

Just a thought,but did you have help bleeding the system or do it by yourself?
It's pretty hard to get all the air out of the system if you use the bottle method,because the stroke on your master is not enough to move fluid from the master thru the whole length of the system.You need to close the bleeder valve after each pedal depression.Unless you have bleeders with a check valve.
Since you said you had a soft pedal it's just a thought.

Good luck, and keep us posted


:seeya :seeya :seeya

piper

Years ago i had a problems like this with a 54 Chev.(us old guys) I had the brakes done at Midas (never again i was in a hurry to get it on the road) before i got the car home(3 miles) the front brakes locked up ,after the car sat for a while they unlocked. Took it back and they worked on it ,took it home same thing(this when on for a week) Asked some of my old friends and this is what it was and how i found and fixed it. When the car locks up jack up the car(I think you said front that is where mine was) Do it as soon as you can ,try and turn the wheels if the wheel or wheels are locked than the brake fluid is not getting back to the master cylinder . My master cylinders return hole was plugged. I took the master out and cleaned it and never had a problem again. Now i see you have tryed new masters, so if it is not that it could be in the block or you could have some poopie in the brake line that is plugging the master cylinder each time. Use a coat hanger and fine the hole while they are locked . I am sure you get the idea .Please let me know if this is it. I have a 67 and i think the caliper are the worest pain in the butt i have ever had

magicv8

The last time I saw the elusive lock up, it was a bent spine on a brake shoe. Sight down the spine of the shoes. If they are not obviously bent, try switching the shoes to the other side of the car and see if the problem moves.

The spongy feeling..... Do you have power brakes?

GATOR2511

Yes I do have power brakes. I have had the brake booster rebuilt with no change in the problem. sorry I'm taking so long to get back but I let my wife talk me into working at the state fair and the hours are very,very long. I'm going back into retirement for the rest of the year after next monday. :D The car has to wait until next week. Thanks Guys :seeya

4_inch_Bore

I checked with an "expert"....
He said he has seen a bad hose do that, and asked if you had blown out the lines before re-installing.
Please let us all know the resolution to the problem, ok?

JohnZ

Are you sure they gave you a drum brake master cylinder? Drum cylinders have a built-in residual pressure valve (about 10 psi) behind the outlet fitting to keep the shoes close to the drums for reduced pedal travel on brake application, and the RPV in disc cylinders is only about 2-3 psi. If you have a disc RPV in it, it would have longer pedal travel before the shoes contacted the drums, and possible uneven brake application unless all the shoe pull-back springs are identical. Just a thought. I was going to ask about the age of the front flex hoses, but you said you replaced everything but the hard lines.