67 Corvette PB J56 Brake Problem

[David S.]

Hi Guys,

I just did breaks on a 67 J56 PB cars. The proportioning valve was rebuilt and the booster is brand new. We bled the front brakes and they are perfect. After pumping the pedal and holding it, the pedal sinks to the floor releasing the air when we opened the bleeder.

The problem is the rear brakes. We have fluid to them, but we cannot get pressure. After pumping on holding the pedal even when someone opens a bleeder in the rear the pedal remains hard and does not sink to the floor as it should, releasing the air.

We swapped a master cylinder, that is not the problem. We also changed the proportioning block, that did not change anything. We pulled the proportioning valve off the car and took it apart again, that looks to be ok.

What are we overlooking? Any help is appreciated.

Best,
Dave

[Domenic T.]

Re: 67 Corvette PB J56 Brake Problem

David,
You will get a lot of responce from this post but start with the back first.

DOM

[Drew P.]

Re: 67 Corvette PB J56 Brake Problem

Hi Guys,

I just did breaks on a 67 J56 PB cars. The proportioning valve was rebuilt and the booster is brand new. We bled the front brakes and they are perfect. After pumping the pedal and holding it, the pedal sinks to the floor releasing the air when we opened the bleeder.

The problem is the rear brakes. We have fluid to them, but we cannot get pressure. After pumping on holding the pedal even when someone opens a bleeder in the rear the pedal remains hard and does not sink to the floor as it should, releasing the air.

We swapped a master cylinder, that is not the problem. We also changed the proportioning block, that did not change anything. We pulled the proportioning valve off the car and took it apart again, that looks to be ok.

What are we overlooking? Any help is appreciated.

Best,

Dave

Hello Dave,
You need to first start with the brake bleeding with the passenger side rear.
The second is the driver side rear.
The third is the passenger side front.
The fourth is the driver side front.
Regards,
Drew Papsun

[David S.]

Re: 67 Corvette PB J56 Brake Problem

Hi guys,

Thanks for the replies. Let me clarify. We started in the rear on the passenger side, then moved to the driver side rear. When we did not get pressure in the rear we moved to the fronts in an effort to further diagnose.

I bled this car the same way I have on every other car I have done brakes on. Its bled by the book.


Thanks,
Dave

[Drew P.]

Re: 67 Corvette PB J56 Brake Problem

Hi guys,

Thanks for the replies. Let me clarify. We started in the rear on the passenger side, then moved to the driver side rear. When we did not get pressure in the rear we moved to the fronts in an effort to further diagnose.

I bled this car the same way I have on every other car I have done brakes on. Its bled by the book.


Thanks,
Dave

Hello Dave,
Did you change the rear rubber hoses? Maybe old rubber hose are the problem.
Regards,
Drew

[John M.]

Re: 67 Corvette PB J56 Brake Problem

Dave, I had a similar problem with a 69 with power brakes. One problem was that one of the brake lines was not completely tight so that it kept sucking air back in, even though the bleeder was closed. I put a Mighty vac on the bleeder and discovered this problem; after fixing that I used the mighty vac to suck fluid through but my final solution was the wife in the car and me at the bleeder saying, down, hold, up.. You know the sequence. Good luck
John McRae 30025

[Rich P.]

Re: 67 Corvette PB J56 Brake Problem

Dave,

I did a 67 435 with J-56 20 some years ago and had the same problem. It turned out to be the setting on the J-56 proportionong valve. If you follow instuctions in the AIM or service manual (I forget but think it is the AIM) it shows which way to adjust the valve by backing off the lock nut and turning the adjuster clockwise or counter clockwise. By doing this you allow for more or less pressure to the rear wheels.

Hope this helps, Rich.

[Claus S.]

Re: 67 Corvette PB J56 Brake Problem

Sometimes when i have bled brakes i have started with the closest caliper to the master cylinder , then the other front wheel before doing the rear calipers just because that have been the only way to get brake fluid to the rear calipers.

Off course when i have fluid in the rear brakes i do the bleeding the correct way with the right rear, left rear, right front and the left front.

[Rich P.]

Re: 67 Corvette PB J56 Brake Problem

A couple more points to concider.
1 make sure your useing the correct 1-1/8" master cylinder
2. make sure the rod is hooked up to the hole in the brake pedal for PB
3 Make sure the clevis fork that screws onto the push rod is for PB not manual
3 make sure the rods are correct for PB and adjusted correctly if you look at how the plunger works in the bore of the master cyl. the rear brakes don't start getting pressure till the pedal is 1/2-3/4 of the way depressed and if you are not getting full pedal to plunger travel you won't get pressure or much pressure to the rear wheels.

If these are all correct then I feel it is either in the adjustment for the J-56 valve or the valve is defective even thought it appears to "look fine in inspection. The valves are troublesome thats why they did away with them in 69.

Ask me how I know about all these problems it's because I have experianced every one of the in the past.

Rich

[David S.]

Re: 67 Corvette PB J56 Brake Problem

Hi Guys,
I replaced the rear brake hoses today. While everything was apart I removed the blocks, cleaned them and blew out the lines. Put everything back together and pressure bled the system. Whatever it was, we solved it. The car has 100% brakes right now.
Thanks for your help,
Dave