I was not sure if my leak was coming from the proportioning valve or the brake line. I thought my front to rear line may have been rusting out. I replaced the front to rear line and rear cross-over line and even put in an new master cylinder.

However, now when I go to bleed the brake lines, I am not getting ANY brake fluid going to the back lines. The fluid in rear reservoir of the master cylinder does not go down when I pump the pedal or when I leave it sit with bleeder screws removed to gravity bleed the system and no fluid is coming out of the rear calipers.

I also bought a new proportioning valve but have not intalled it yet.
However, I don't think the part everyone is selling claiming to be a proportioning valve is a valve. I think it is just a distribution block.

There is another brass piece on my 74 proportioning valve which is round and about 1 1/2 - 2 inches long which the front to rear line screws into ,which then screws into the rectangular brass piece that the other lines go into.

Is this the piece causing my problem of not getting brake fluid to the rear of the car?

Is this piece available either re-built or new OR can I fix mine??

Please help, I need to get this fixed soon, I have spent way too much time messing with this.

Thanks for any help you can provide.

 

I'm no brake expert, but I to had a '74 with a leaking proportioning valve. The little spring washer was missing and it was leaking from that little rubber cap. I replaced the whole valve and have not had any problems since.

Word of advice (ask me how I know), find a way to cap the lines running from the master cylinder or you will have brake fluid everywhere.

Also, I gravity bled the brakes afterward and it took upwards of twenty or thirty minutes to get a steady flow at the rear brakes.

I'm not sure this would be related to the bleeding problem but did you bench bleed the master cylinder before installing?

Hope some of this helps.

 

Follow your brake lines from the master cylinder to the drivers side frame rail. The first component the lines go to is the proportioning/combination valve, it has input and output lines to both front and rear brakes plus an electrical connection to illuminate brake warning light on the dash. Beyond that all other valves are distribution blocks, with no electrical connections.

If you have no fluid in the rear system it means that the proportioning/combination valve has internally shifted to protect the system. Had you lost rear pressure while driving then the valve shifting internally ensures that you maintain only front brake pressure to safely stop the vehicle, without completely depleting the brake fluid. At this time the dash warning light would have illuminated to alert the driver of the brake problem, in the event that the driver didn't notice the lack of braking.

In order to reset the proportioning valve, you must have fluid on both sides of the valve and make a hard brake application to recenter the internal sleeve/piston. If properly re-centered the dash light should go out.

To get fluid on both sides of the valve you need to either gravity feed the rear brakes or use a vacuum system. The difficulty with trying to bleed the brakes after changing a brake line is that the proportioning valve will internally shift as soon as you apply brakes, because one half of the system has no fluid. By using gravity feed or vacuum you fill the system without shifting the valve.

I would suspect that there is nothing wrong with your proportioning/combination valve. Changing it still leaves you with the same problem of trying to get fluid in the rear system.

 

If your valve looks like this, it's a "Proportioning Valve"



If it looks like this, it's a warning switch (distribution valve) only.



Loosen the fitting to the rear brakes at the MC to see if it will gravity bleed from that point. If so, move down stream until you find the problem. Make sure the MC piston is coming all the way back. Loosen the 2 mounting nuts and pull it forward to make sure. Not much can go wrong with either valve to cause it to not flow fluid. With a dry system like you have, it makes it easier to bleed if you pull a vacuum at the bleeder screw or pressure bleed (Motive Bleeder tool) from the MC. Check also the rubber hoses as they can go bad causing restriction and bleed issues. If you have the proportioning valve, I'd go ahead and replace it as they are a possible leak point. (see this post)

http://forums.corvetteforum.com/show....php?t=1830792

 

PM rec'd.

PM sent.

Good luck, Dan

 

I just recently went through this problem. My rear brakes hose swoll up causing the P-Valve to shift foward. I could not get any fluid to the rear lines. 1st I changed the rubber hoses. I knew that these were bad due to not being able to blow air through them. While waiting on the hoses, I took my P-Valve out and reset it manually. If you remove the switch in the top, you can see the plunger in the block. If your brake light is not on, verify by taking an ohm meter connected to the center prong of the switch before removal. Connect the other lead to the block. If the ohm meter shows zero resistance your plunger has shifted, & your light might be burnt out. If it shows open your plunger is in the correct position.

Last but not least buy or borrow a pressure bleeder from someone. You can bleed the brakes buy yourself, and not cause the P-Valve to shift foward. I had to wait a week for it, but its money well spent!

 

Thanks for everyone's help.

I will work on it some more tomorrow and I should have it fixed!