Brake Bleeding Problem 1965

[Joe C.]

Re: Brake Bleeding Problem 1965

Dual circuit master cylinders have a lot of rear bias. My guess is that the system has to be pressurized in order to have fluid ejected from the front feeding port.

[Terry M.]

Re: Brake Bleeding Problem 1965

Dual circuit master cylinders have a lot of rear bias. My guess is that the system has to be pressurized in order to have fluid ejected from the front feeding port.

Actually that is true up until the X-car of the early 19780s. After that the rear brakes, at least on GM cars, almost don't do anything. Typically the after 1980s brake systems are biased 70% to 80% to the front.

Is this master cylinder part of a conversion kit?

I ask because master cylinders designed for use with front disks and rear drums use a metering valve that does not allow front pressure to build until rear pressure reaches a certain minimum level. The idea is to get the rear drums in action before the front brakes apply. Did you ever put on the front brakes on your bicycle before the rears?

Sometimes that metering valve is a separate unit and sometimes it is part of the master cylinder.

[Carl B.]

Re: Brake Bleeding Problem 1965

if you purchased a master cylinder rebuild kit with new pistons the pistons are not correct you will have to use the old pistons the correct pistons have small holes in the front piston the kits you purchased do not just replace new seals on old pistons had my master apart many times on my 66 before I checked real close

[Frank D.]

Re: Brake Bleeding Problem 1965

Joe:
What do you mean by "pressurized."? How is that done?

Terry:
The master cylinder is not part of a conversion kit. The car originally came with the dual port master cylinder. We installed a rebuild kit for the master cylinder since everything else in the system was being replaced as well.

Thanks for staying with us on this issue!

Frank O'Donnell

[Terry M.]

Re: Brake Bleeding Problem 1965

OK, I'll surrender on my lack of understanding of the master cylinder configuration on a 1965. I am out of my element on that issue, but brakes are brakes -- more or less.

If the front pistons don't move you are building no pressure in the front. This could be because of the master cylinder (one possibility is what Carl has suggested. There also could be a blockage in the master.) or there could be a blockage in the line(s) to the front or a blockage in that distribution block you mentioned.

It would make a mess, but loosening the fitting that feeds the front at the master cylinder and applying pressure to the brake pedal would give an additional data point. Keep a shop rag around the fitting and plenty of water handy. The spilled DOT 3 or 4 (DOT 5 will not have that issue) will remove paint if not diluted and cleaned up quickly.

If that fitting shows a volume of fluid, tighten it back up and move down the line until you get no fluid flow and you have found your blockage.

If you can get a pressure bleeder (John Hinkley has posted pictures of a rig he recommends) you will eliminate the need for two people, and might prove the master piston confusion that Carl mentioned.

[Joe C.]

Re: Brake Bleeding Problem 1965

Joe:
What do you mean by "pressurized."? How is that done?

Terry:
The master cylinder is not part of a conversion kit. The car originally came with the dual port master cylinder. We installed a rebuild kit for the master cylinder since everything else in the system was being replaced as well.

Thanks for staying with us on this issue!

Frank O'Donnell

It goes back to what Terry said about pressure first being delivered to the rear before the front, and is a part of the rearward bias that I referred to earlier. Cars with dual circuit master cylinders are metered to have a slight delay in delivering pressure to the front brakes; they are also biased rearward. Safety issue since locking the fronts before the rears results in total loss of steering control. ALL cars are built with this bias. Stand on the brakes of any car with dual circuits, correctly metered, and without ABS. It will be impossible to lock the fronts before the rears, unless the front tires have severely limited traction.

You must pressurize the entire system, as it was designed to work, before any working fluid will be delivered through the front feeding port. You can probably rig up a bench test stand, but it's a lot easier to do it on the car. Short of that, is to remove the offending caliper, and pressurize it on its own, in a holding fixture.

[Frank D.]

Re: Brake Bleeding Problem 1965-Update Again

Carl: Thanks for joining in. We matched up the old pistons with the new ones before we installed them on all four calipers, and the rears both bleed when tested.

Terry: We just tried your advice and fluid is moving directly through the left front caliper and all four pistons show movement. We had fluid pressure from the front port of the master cylinder yesterday, so we are checking the t-block (located on the top of the frame) for any blockage now.

Thanks again,

Frank O'Donnell

[Paul H.]

Re: Brake Bleeding Problem 1965

I see in your initial post that you changed the lines. I have had rubber lines (even new ones) that partially collapsed and caused an obstruction in the line preventing an even flow to the caliper. Just a thought.

[Frank D.]

Re: Brake Bleeding Problem 1965

Paul:

Thanks for your input. We'll re-check all the front lines including the crossover.

Frank

[John H.]

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

[Joe C.]

Re: Brake Bleeding Problem 1965

post deleted at request of poster - Joe Ciaravino (32899)

[Frank D.]

Re: Brake Bleeding Problem 1965--UPDATE/Finish

ALL: The brake lines all checked out, with no blockage or crushed lines. We tried everything all over again, ran fluid every-which way and viola! brake fluid and pressure. I wish I could tell you exactly what fixed the problem, but we think there was LOTS of air trapped evreywhere and it just took us the right combination of tactics to get it all out: two-man pumping, gravity feeding and bench bleeding. We won't even go into the details of using of a wet-dry vac to create some extra suction. In the end it was exactly what one poster offered as encouragement..."be vigilant and persistant and don't give up."

At any rate, thank you all for your help and staying with us as we fuddled through this problem.

Frank O'Donnell

[Terry M.]

Re: Brake Bleeding Problem 1965

I am glad you worked through it, and that it wasn't anything major.

Thanks too for posting back. Learning here is a two-way street and it helps greatly to learn what worked for you.

[Michael G.]

Re: Brake Bleeding Problem 1965

Thanks for the atta boy Frank......as stated "stay vigilant, patience will win in the end". I love happy endings.....think I'm going to tear up.

[Frank D.]

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)