Troubleshooting brake bleed

[Charles P.]

The brakes on my 73 were down a bit with a newer set of calipers and master cylinder done about 2 years ago. Probably less then 1500 miles since then. I brought the car down to the local Chevy dealer to see if they could bleed the system. No brake fluid visible anywhere so I wasn't thinking caliper. They seemed to have a hard time bleeding the system and after 2 plus hours the brake was non existent and impossible to drive the car home. They were getting good flow from the front brakes but when they opened the backs it was little more then a spit and air. They tried this repeatedly. They are unsure what the problem is. One person mentioned Master cylinder while another said possibly the proportioning valve. Still another said it should be bled using the brake fluid in a bottle method/tubes to prevent air going back into the system when they opened the bleeders.
What could cause no flow to rt (or left) rear when bleeding? Any ideas what they are doing wrong? Thanks.

[Oliver B.]

Re: Troubleshooting brake bleed

...it should be bled using the brake fluid in a bottle method/tubes to prevent air going back into the system when they opened the bleeders...

Isn't this the standard procedure, anyhow?

Then the line(s) to the rear calipers might be blocked. This does happen not too rarely, when the cars don't get driven and/or the fluid is not changed on a regular basis.

But shouldn't your Chevy dealer know about both? Those are not even Vette-specifics...

[Alan S.]

Re: Troubleshooting brake bleed

Hi Charles,
I had a similar situation on my 71. Using a Miti-Vac I could only get air or nothing at all at the rear calipers. I could hear the Mity-Vac pull the rear pistons back against the springs. I was unsure of the original master cylinder so decided to replace it with the reproduction. I then could bleed the system normally. I don't know enough to understand if the master was the problem or if it was a 'back door' solution.
Good Luck!
Regards,
Alan

[Charles P.]

Re: Troubleshooting brake bleed

Isn't this the standard procedure, anyhow?
But shouldn't your Chevy dealer know about both? Those are not even Vette-specifics...

I do alot of things I can reach. Once in a great while I will go to someone when it is hard to reach or cost effective but lately it is as if they find the older cars harder to work on. My money is on the master cylinder. How often does a proportioning valve go bad and how does that one work on a C3?

[Terry M.]

Re: Troubleshooting brake bleed

Do 1973s have a proportioning valve? 1972s do not, and I suspect 1973s don't either -- but I don't know squat about 1973s.

It kind of gets me when people, especially "trained" techs refer to the C3 pressure differential valve and distribution block as a proportioning valve. For trained technicians to make this mistake they are demonstrating their ignorance of the fundamentals of automotive brake systems.

[Michael W.]

Re: Troubleshooting brake bleed

Do 1973s have a proportioning valve? 1972s do not, and I suspect 1973s don't either -- but I don't know squat about 1973s.

No proportioning valve on '73s, it's the same distribution block/warning switch widget as earlier C3s. Actual proportioning valves started in the late 70s somewhere.

Frequently, the piston in the earlier type valves is out of position far enough to obstruct flow to one end of the system or the other. For reasons that are beyond my comprehension, the rear brakes are more frequently affected. The OPs mentioned that the brakes were 'down a bit' perhaps this was the inititation of the problem

The first task is to free the stuck piston, recenter it and then process with the bleeding.

Charles, is the brake warning light 'on'?

[Terry M.]