Brake Bleeding Problem 1965

[Joe L.]

Re: Brake Bleeding Problem 1965

Joe -

Not true. 60's GM cars with dual-circuit disc/drum brakes have a separate external metering/hold-off valve that denies fluid to the front discs until the system sees 30-40 psi, to allow the rear shoes to expand out to the drums first, to prevent disconcerting "nosedive" during braking, primarily at lower speeds. Those cylinders also use a residual pressure valve in the outlet port that serves the rear drum brakes to hold the shoes off their return stops. There is no biasing or metering of any sort internal to those master cylinders.

60's GM dual-circuit master cylinders for Corvette disc/disc usage have no metering or biasing internal to the master cylinder, and don't utilize an external metering/hold-off valve, which isn't necessary with discs on all four wheels.

Further, the primary design objective for all GM non-ABS brake systems is to ensure that the front brakes ALWAYS lock up first under maximum-effort panic braking, so the rear wheels continue to rotate and track straight behind the path determined by the front wheels. If the rears lock up first, they don't rotate, don't trail the path of the front wheels, and the car will spin. That's automotive Brake Engineering 101, and is the first thing Brake Engineers learn.

John Hinckley

John------


Exactly. For a 1965-77 Corvette there is no metering or "proportioning" internal or external to the master cylinder. The "bias" between front and rear is accomplished by the different size pistons used for the front and the rear.

With the "weight gains" that Corvettes experienced starting in 1978, a proportioning function (fixed rate) was built into the brake distribution block and differential pressure switch. I believe this was sort of a "band-aid" fix and done as an alternative to resizing the front and rear caliper pistons. This part was used for all 1978-82 Corvettes.

[Joe L.]

Re: Brake Bleeding Problem 1965

It's baaack. Last Friday, after finally getting good pedal pressure through a mariad of bleeding proceedures, we pulled out of the garage for the test drive and was stopped in our tracks by a major power steering leak.

After two more days of rebuilding and testing the power steering, we give up and install a new power steering control valve. New problem solved. We pull out of the garage to again test the power brakes and find that the brake pedal goes all the way to the floor: no brakes. We bleed again successfully and we have good pedal pressure, (with the engine off) but when we start the engine the pedal pressure disappears and the brake pedal goes right to the floor. It is almost like the system is working backwards, i.e. when the engine is started up the pedal pressure goes away.

So, it appears we didn't fix the problem after all. Please send us your ideas to try to fix the problem.

Thanks again in advance,
Frank O'Donnell
Troy, VA
(Car is in a home garage in Charlotte, NC)

Frank------

I assume that you haven't tried a Motive Products pressure bleeder? It might solve your bleeding problem assuming the problem is just one of incomplete evacuation of air from the system.

Getting all of the air out of a 1965-82 Corvette brake systems can be one of the most frustrating and difficult jobs on such a Corvette. I dread it almost more than anything else.

[Frank D.]

Re: Brake Bleeding Problem 1965--Late P.M. Update

We switched front and rear brake line connections to the dual reservoir master cylinder and now have REAL pressure to the front discs. We'll test the car in the morning when we can actually see what we might be colliding with.

It would appear the front half of the master cylinder is where the problem lies. Thanks to Bob Young of the Carolinas chapter for that suggestion. At least we know it was'nt a problem with our air bleeding techniques.

We are still taking suggestions, though!

Frank O'Donnell

[Joe L.]

Re: Brake Bleeding Problem 1965--Late P.M. Update

We switched front and rear brake line connections to the dual reservoir master cylinder and now have REAL pressure to the front discs. We'll test the car in the morning when we can actually see what we might be colliding with.

It would appear the front half of the master cylinder is where the problem lies. Thanks to Bob Young of the Carolinas chapter for that suggestion. At least we know it was'nt a problem with our air bleeding techniques.

We are still taking suggestions, though!

Frank O'Donnell

Frank------

I thought that Carl had hit on the problem with his post regarding the possible problems with the replacement master cylinder pistons. Then, you said that you had compared the new and old pistons and they were the same. Well, after your latest post it sounds to me like you have a master cylinder problem. If I were you, I'd disassemble the master cylinder and carefully compare the new and old pistons. I doubt VERY seriously that a problem suddenly cropped up in the master cylinder casting. So, if there is a master cylinder problem, that means the problem has to lie with the pistons or the seals.

[Frank D.]

Re: Brake Bleeding Problem 1965

Thanks for the reply Joe. We are also thinking it might be the master cylinder and the rebuild that we did. We will check that tonight.

This morning we test drove the car with the lines to the dual reservoirs reversed and had good pressure. After unplugging and blocking off the brake booster vacuum line, the brakes were even better, but it appears we only have front brakes.

Still looking for theories though, in case the master cyclinder checks out ok.

Thanks again,
Frank

[Frank D.]

Re: Brake Bleeding Problem 1965-"FINAL ANSWER"

Thank you to all who posted theories/ideas for us try. Here was the final answer. If you recall, my car is a '65Coupe with power brakes and the factory-installed dual master cylinder with the two screw-in milk caps.

After five days of bleeding (literally), pumping and cursing I trailered my car back home to Virginia. At the very end, we had 20% brake pressure; and if the booster hose was hooked up with the engine running, the brake pedal went all the way to the floorboard giving 0% brake pressure.

I took the car to my favorite mechanic who took the wheels off the car and promptly correctly identified the problem. On both rear calipers, we had the bleeder screws inserted front facing and rear facing; when we should have had the bleeder screws front facing outboard and front facing inboard. In other words, we had the block-off bleeder screws inserted into the inboard bleeder slots. So we had successfully trapped air inside both rear calipers no matter what how well the front calipers could be bled.

So, all of you are thinking, "I would have taken pictures of those calipers if there was any chance I might be rebuilding them." But anyway... there is our answer. The booster, master cylinder and all the calipers were rebuilt correctly, it was just where the bleeders were placed that was causing the whole problem.

Thanks again for all your efforts to help out. You guys are the best.

Frank O'Donnell
Charlottesville, VA
See you in Charlotte!

[Joe L.]

Re: Brake Bleeding Problem 1965-"FINAL ANSWER"

Thank you to all who posted theories/ideas for us try. Here was the final answer. If you recall, my car is a '65Coupe with power brakes and the factory-installed dual master cylinder with the two screw-in milk caps.

After five days of bleeding (literally), pumping and cursing I trailered my car back home to Virginia. At the very end, we had 20% brake pressure; and if the booster hose was hooked up with the engine running, the brake pedal went all the way to the floorboard giving 0% brake pressure.

I took the car to my favorite mechanic who took the wheels off the car and promptly correctly identified the problem. On both rear calipers, we had the bleeder screws inserted front facing and rear facing; when we should have had the bleeder screws front facing outboard and front facing inboard. In other words, we had the block-off bleeder screws inserted into the inboard bleeder slots. So we had successfully trapped air inside both rear calipers no matter what how well the front calipers could be bled.

So, all of you are thinking, "I would have taken pictures of those calipers if there was any chance I might be rebuilding them." But anyway... there is our answer. The booster, master cylinder and all the calipers were rebuilt correctly, it was just where the bleeders were placed that was causing the whole problem.

Thanks again for all your efforts to help out. You guys are the best.

Frank O'Donnell
Charlottesville, VA
See you in Charlotte!

Frank------


I'm sure a lot of us could have identified the problem if we had been able to see the calipers, in person or with photos. A problem like you described would be practically impossible to determine without actually seeing the parts. It's just not a typical or "predictable" sort of problem.

Now that the bleeder valve situation is corrected, have the brakes been successfully bled with a firm pedal?

[Frank D.]

Re: Brake Bleeding Problem 1965

Good point, Joe. I will remember that in the future.

The brakes were bled at my mechanic's shop with his power bleeder. They were out test driving the car before I even knew he had found the problem. Tomorrow I will do the five stops in reverse to set the brakes.

Frank