1969 C10 Disc Brake Conversion, Only Front Brakes will Work???

[matthew20061]

While restoring my 69' C10 which originally had manual 4 drum brakes, I converted the front brakes to disc brakes, new calipers, rotors, pads, and hoses. I bought a new master cyl from Car Quest for a 72 Chevy with factory front disc brakes. Then I got a proportioning valve from CPP. Before installing the master cyl I bench bleed it very good so that no bubbles were coming out. After plumbing new lines from the new master cyl to the proportioning valve, I bleed all the brakes starting with the passenger rear, drivers rear, moved my way up to the front passenger side and driver side. So after pressure bleeding them, I only had front brakes. So I gravity bled them and slide the prop valve back to center and tried them again. Same thing instant only front brakes. Its been this way for the last year. I've gone thu like a gallon of brake fluid or more just in bleeding these things so I'm almost 110% sure theres no air in the lines. Brake pedal is also hard, but when I slide the prop valve back to center and jamb something in there to keep it centered they feel great just like normal. But it always slides back over and blocks off the rear brakes???

I have access to an original working master cyl and original factory proportioning valve from a 72 Cheyenne should I swap this on there and see if it fixes it?

So what could cause this? Bad master cyl? Bad proportioning valve?? Or?

[mclairmo]

Have you called CPP or e-mailed Jeff from CPP who's on here quite frequently? I don't think that prop valve is supposed to block the rear brakes under normal circumstances. I believe that's a safety device intended to cut off a section of the brakes if a massive leak occurs so as to preserve at least half your brakes. Might be a defective prop valve.

[matthew20061]

Originally Posted by mclairmo

Have you called CPP or e-mailed Jeff from CPP who's on here quite frequently? I don't think that prop valve is supposed to block the rear brakes under normal circumstances. I believe that's a safety device intended to cut off a section of the brakes if a massive leak occurs so as to preserve at least half your brakes. Might be a defective prop valve.

No I haven't contacted him, I didn't even think about contacting CPP. Maybe I'll give them a call sometime.

[cdowns]

the one thing i noticed you did wrong was the driver rear is the furthest away and should b bleed first

[Tx Firefighter]

Are the rear brakes adjusted up nice and tight ? If they are too loosely adjusted, the volume of fluid that is needed to push them out against the drum can trigger the prop valve by mimicking a rear brake failure which would cause the issue you're having.

[matthew20061]

Originally Posted by Tx Firefighter

Are the rear brakes adjusted up nice and tight ? If they are too loosely adjusted, the volume of fluid that is needed to push them out against the drum can trigger the prop valve by mimicking a rear brake failure which would cause the issue you're having.

Yes I have tried adjusting them many times with no luck, I think I may have just found my problem in this thread: http://67-72chevytrucks.com/vboard/s...d.php?t=241722

[matthew20061]

Originally Posted by matthew20061

Yes I have tried adjusting them many times with no luck, I think I may have just found my problem in this thread: http://67-72chevytrucks.com/vboard/s...d.php?t=241722

Well I just tried the fix in the above thread with no luck still have no rear brakes unless I center the prop valve, and as soon as I hit the brakes it blocks off the rear brakes.

[PICKMUP]

Like you said...try another one and eliminate that area.

[matthew20061]

Bump, any other ideas?

[MRcamaro]

swap out the valve... process of elimination.

[Tx Firefighter]

I figure he has found a solution. The thread has been dormant for three years.
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[MRcamaro]

I would hope so but he said it was previously like that for a year before he tried fixing it and seems like he didn't want to swap out the valve, would like to know what he did about it just in case I ever have this problem.

[Tx Firefighter]

That's one of the bad parts about boards like this. A lot of guys never follow up with final outcomes. We are sometimes just left wondering what the answer came out to be.
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[matthew20061]

Originally Posted by MRcamaro

I would hope so but he said it was previously like that for a year before he tried fixing it and seems like he didn't want to swap out the valve, would like to know what he did about it just in case I ever have this problem.

Glad someone bumped the thread totally forgot about it. I ended up replacing the proportioning valve with a new one and it solved all of my problems. The CPP one must have had something bad inside of it. After the change I rebleed the brakes and everything works perfectly just like it should! Sorry I never followed up.

[MRcamaro]

Cool, thanks for the follow up. Will be that much easier if this problem comes up.
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