This is somewhat of a lenghty explanation. But I'm tired of "spinning my wheels" (and not getting them stopped !). So I'd appreciate any advice/answers/suggestions from the experts here.

A few years ago, I had brake problems with my '69 Buick SportWagon (A-body). It came from the factory with disk brakes. I believe that all '69 A-Bodies that came with disk brakes also had a special "metering valve" located next to the master cylinder. This valve has a plunger on the back end (the manual says to depress the plunger when bleeding the brakes). I believe that after '69 this metering valve was eliminated and the plunger was incoproprated into the proportioning valve on the frame.

Question #1: Are the '69 disk-brake and drum-brake proportioning valves the same ?? (I know that they are different in later years).

The problem with my Buick was that no matter how much I bled the brakes, air bubbles kept coming out, but no fluid was leaking. It turned out that the metering valve was bad. I had it rebuilt and the problem was cured.

Which brings me to me current problem - My '68 El Camino. When I bought it 12 years or so ago, it had single-piston disk brakes on it (whch were not produced until '69). But the car came with various extra parts including several from a '69 El Camino. So I'm guessing that the previous owner swapped the disk brake setup from a '69. It has exactly the same metering valve and proportioning valve as my '69 Buick.

Anyway, a few weeks ago while idling at a stop light, I noticed the brake pedal go soft and it crept to the floor. That was similar to what happened with my '69 Buick.

I have been running silicone (Dot 5) brake fluid in my El Camino for many years now. When I took the master cylinder off the car and poured out the fluid, it looked like there may have been some traces of the old fluid still in there - and it was dirty. Whatever it was, it looked like oil and water (two kinds of fluid that don't mix).

This time I didn't want to mess around, so I sent the master cylinder and metering valve to a brake place (that advertises in Hemming's) for a rebuild. I installed the rebuilt parts, but I was unable to bleed the front brakes without bubbles coming out. I suspected that the metering valve was still bad, so I bypassed it with a brass coupler - still had bubbles. I had my assistant hold the pedal. I'd crack open the bleeder, and then close it when the pedal got near the floor. After cycling a couple bottles of fluid through the system, I still had bubbles coming out. This was the case at both front wheels. A fair amount of fluid was coming out when the brake pedal was floored, but hardly any was sucked back up when the pedal was released (I was using a hose that was connected to the bleeder valve at one end, and submerged in a jar of fluid at the other end).

Then I tried the back wheels. I was unable to get ANY pressure whatsoever at either rear wheel. I don't know how long that condition may have existed. Here is a diagram of my brake system:

http://www.DesignsComputed.com/images/brakes.jpg

The wrench size for each fitting is noted as 7/16" or 9/16". The smaller-diameter brake lines are noted as a single line and the larger-diameter brake lines as a double line.

I verified that there is pressure coming out of the master cylinder at fittings "A" and "B".

Question #2: Why is there no brake pressure at the rear wheels (is there a blockage somewhere, or is the proportioning valve bad) ?

Question #3: Is it possible for a proportioning valve to let air into the system even if no fluid is leaking (like what happened with the metering valve on my '69 Buick) ?

Question #4: If my proportioning valve is bad (or suspect), where is a good place to get a replacement '69-style proportioning valve ?

Last Question: Is there anything I'm not doing right in the bleeding process or elsewhere ?

Thanks for any help !

I ASSume you bench bled the master cylinder BEFORE installing it correct? See the DOT5 discussion that was just posted a few days ago.

Originally posted by dennis68:
I ASSume you bench bled the master cylinder BEFORE installing it correct? See the DOT5 discussion that was just posted a few days ago. Thanks, I looked over that discussion. The spongy pedal in my car came on fairly suddenly - it wasn't that way before my recent problems. In fact, it was fine with the DOT 5 for about 8 years, at least.

No, I didn't "bench bleed" the master cylinder first. What I did do was this:

I mounted it on the car.
Ran rubber hoses with plastic fttings from the MC outlet ports to the reservoirs.
Filled it with fluid.
Pumped the pedal until there were no more bubbles cycling.
Then I attached the metal brake lines to the MC and started the normal bleeding process.

What does the bench bleeding do, exactly ?

It looks like you did bench bleed it when you mounted it on the car and connected the rubber hoses

Bench bleeding gets the pockets of air out of the master cylinder.

Have you had the calipers off of the car? Make sure that the bleeder screws are pointing upwards. Also check the fitting at the calipers to make sure the copper washer is sealing.

Chris.

Dan,
On our cars the M/C sits at an angle and that makes it easy for an air pocket to form in the front of the master.Bench bleeding helps to eliminate this.Since you've already installed it,you can jack up the rear till the master sits level and rebleed it.
The best way to bleed the brakes that I've found is whats called surge bleeding.Basically what you do is you open the bleeder then your helper pushes the pedal to the floor and holds it there.After you close the bleeder,helper releases the pedal.You repeat this till the bleeders stop spitting air.
This method helps to avoid pumping the fluid into a frothy mess,which if your using DOT 5 silicone it is easy especially susseptable to this.One other thing I use when a system is air bound is a brake fluid syringe.

Heres a syringe bleeder
http://www.mpbrakes.com/ac20010.htm
I got mine from a parts store that carries EIS brake products,but they've been bought out by someone like Wagner or Bendix.These things work great when a system is air bound.