Brake Proportioning Valve problems.
 

I have replaced all the brake parts and after gravity bleeding it all I started it up and the first push on the brakes turned on the great light we all like to see. I checked the reset button and it was not poped out so I tried the bleeder on top of the Prop valve and alot of fluid came out but now I get nothing out of any of the wheels. Whats going on with this thing. It is a 70 with disk brakes and I have no idea if it is the right valve.

Any input would be great.

Kevin
LFD Inc.

Re: Brake Proportioning Valve problems.
 

Did you bench bleed the MC?

Re: Brake Proportioning Valve problems.
 

I had the same problem before. with a buddy around (to close all the bleeders) crack open all the bleeders, then slam the brake pedal to the floor fast and hard. while holding the pedal down, have the other guy close all the bleeders. pump it a few times and bleed normally. sounds stupid but it works

Re: Brake Proportioning Valve problems.
 

PS, my above post is taking into account you DID bench bleed the master. don't do the above if you didn't, cause it will ruin the seals in the master...

Re: Brake Proportioning Valve problems.
 

I did bench bleed the master, gravity bleed and can bleed all four corners and had a good pedal. I then fired it up and hit the brakes and the light came on. I checked the pop out button and it was not poped so I had my wife pump and hold the pedal while I cracked the bleeder on top of the pictured Prop. valve. It bleed out like a normal bleeder does but then that was it for any fluid out of any corner. The pedal will not pump back up or anything:confused:
Does it look to be the correct Prop valve for a disk/ drum set up or is this the drum/drum Prop valve? It was on the truck when I got it and so were the front disk so I have no info on what they were off of or what parts were left un touched.

Kevin
LFD Inc.

Re: Brake Proportioning Valve problems.
 

See if this will help:
http://www.classicperform.com/TechBo...t.htm#testprop

Re: Brake Proportioning Valve problems.
 

Re: Brake Proportioning Valve problems.
 

Kevin, I had the same issue recently with a new disc brake conversion on my 70' truck. I was worried because the light came on after the install. I drove down my street nice and easy and "tapped" the brakes a few times and heard a "pop" which was the valve recentering itself and the light went out:) . I've seen posts on here to slam on the brakes but I didn't have to do that. I think you need to take it for a spin and let the weight of the truck coming to a stop recenter the valve. Good luck!

Re: Brake Proportioning Valve problems.
 

you need to use a clamp to push the pin in on the combination valve while bleeding the brakes for proper operation

Re: Brake Proportioning Valve problems.
 

Maybe a stupid question, but what pin, I have not heard of it (don't mean a whole hell of a lot) but I'm gathering all of my disc brake parts up to do mine soon and any and all info would be great.

Re: Brake Proportioning Valve problems.
 

theres a pin at the front of the combination valve ,sometimes its covered by a rubber bulb//theres a special gm tool for holding it in while bleeding brakes but a cclamp will work// the service manual has good pics and description of the process

Re: Brake Proportioning Valve problems.
 

Well I got so fed up with this last night that I puled off the booster and put on another one I had and did the same with the master. I pulled the psi switch out of the Valve and re-centered it but it keeps flipping and shutting off the rear brakes. I even tried holding the valve inplace while bleeding and still nothing. I tried holding in the pin thingy while doing it but nope nothing still. When I hold the valve centered I get fluid out o the rear but as soon as I try it with out holding the valve it flips:confused: I am temped to pull the P-valve and just run an adjustable to the rear if I can not figure this thing out. I pulled the rear drums off and made sure they were not leaking and that the shoes were up tight to the drums and every thing looks fine back their. We have all the air out and have flowed a few bottles of fluid thru the system.
The worst part is I know what I am doing but I can't figure it out. I even went as far as finding my old chassies and brake books from school (20yrs ago) and re-read them for what I was missing and still nothing.
So now what ?

Kevin
LFD Inc.

Re: Brake Proportioning Valve problems.
 

"slamming" the pedal to the floor equalizes pressure in the front/rear and will center the valve. try it.

Re: Brake Proportioning Valve problems.
 

That did not work!

Kevin
LFD Inc.

Re: Brake Proportioning Valve problems.
 

Have you replaced the rear brake hose?I'm just throwing a guess out there but poor flow thru that hose "may" make the prop valve act up.

Re: Brake Proportioning Valve problems.
 

The only part of this system that is original are the front hard lines. Every ruber line and hard line have been replaced as well as all bolt on brake parts.

Kevin
LFD Inc.

Re: Brake Proportioning Valve problems.
 

Check the routing of your 2 lines going from m/c to prop.valve. They usually run different on a 1/2 ton vs. a 3/4 ton. I think you have them routed like a 3/4 ton.

Re: Brake Proportioning Valve problems.
 

Sorry,I was wrong . I had them mixed up. I went to look at one and you have them right.

Re: Brake Proportioning Valve problems.
 

One thing that is funny is I always thought that the small rear bowhl of the MC was for the front disk brakes since they ride on the rotor and need little fluid movement. On mine the small rear bowl is routed to the rear drums. Is this right?

Kevin
LFD Inc.

Re: Brake Proportioning Valve problems.
 

I think your correct, this is true for most vehicles. heres a pic of of mine, I dont know if it will help considering Uncle's post.... mine is a 3/4 ton going to be a 1/2. ??

Re: Brake Proportioning Valve problems.
 

The front bowl is for the front brakes for front disc brakes - consider the size of the pistons in the front calipers. As the pads and rotors wear, it will take a tremendous amount of fluid to displace the front pistons with their large bores compared to the relatively small pistons in the rear wheel cylinders.

Re: Brake Proportioning Valve problems.
 

not only that, butthe disks use fluid to hold the adjustment, the rears use mechanical adjusters.
The large res is always for the disks, and the small is for the drums.

Re: Brake Proportioning Valve problems.
 

All Fixed!!:rolleyes:

Well I had enough so I went to the auto shop accross the street from my house to pick their brains. No luck! I had already tried every thing they said to do and even more. But they gave me the number of a power brake shop in Sacramento. I called and spoke with Sandy and told him everything I had tried and he was impressed with the extent I went to to find the problem. He then said if I had replaced the front rubber lines that I still have air in them. I said I fent I had got it all out but I was willing to try anything. So he then explained that the master cyclinder can not push enough volume of fluid to get the volume of air out of the front lines. He said to wire the front lines down to the frame so that they were lower than the bleeder's. So I did that and I also jacked up the A arms as far as I could to help out. I then put a bottle bleeder ion the raised up bleeder and pumped the pedal about 4 times untill the bottle was full and repeated for the other side.

With my fingers crossed I pulled the screw driver out of the PSI port (used it to hold the valve from flipping while bleeding) and pushed lightly on the brake pedal and it stayed in place. So I then jumped on the pedal a few times and it stayed in place so I put the PSI switch back in and started it up and jumped on the brakes again and I have the best feeling pedal I have ever had in a vehical. The rear end is up on stands with the tires off but lugs holding on the drums so I put it in gear and hhit the brakes and guess what, they stopped:hm: I tried this last night and could not even stop the rear drums at idle.

Well thanks to all who tried to help and now I have learned a new little (very simple) trick.

Kevin
LFD Inc.

Re: Brake Proportioning Valve problems.
 

I have been following this as I have been fighting this exact same problem and have pretty much mirrored all the steps you have gone through to resolve the problem, with the same results you have had. I replaced the left front hose about the same time I replaced the master cylinder and booster so I think I know now why I can't get the light to stay out or the LF wheel to bleed. I can't wait to try your new trick, I bet it will work for me too.
Good work Kevin!

Jeff

Re: Brake Proportioning Valve problems.
 

This needs to go to FAQ when everyone has seen it. Please remind me to move it.:D