69 c20 disc brake help

[treveiger]

Hey guys Ive got a few questions. The PO of my truck put front disk brakes on from a 71-72 chevy c20. The problem is my brakes seem soft and when I push the peddle all the way to the floor itll stop but not fast, and I cant lock them up at all. I checked the brake lines and Im pretty sure its 1/4 for the back brakes and 3/16 for the front. Im going to replace a few hard lines/rubber ones when I get the chance but Im just trying to figure out if theres anything on the truck that you notice that isnt right. Like if my MC isnt right or my Prop. Valve. Im stumped so Im hoping you guys can help me out. O ya before I forget, I changed my pads on my front disks yesterday and when I went to "pump" the brakes to get pressure back on the rotors I got nothing, I messed around with the MC a while and with the rubber on the lid and finally got it to at least make it so I can slowly stop. Ive changed a few brakes out before and never had that happen, I about threw up because I was about to take it for a spin and could of only imagined what would of happened....

[lolife99]

Looks like a drum brake M/C and distribution block.
I think you need to buy a new M/C and a disc brake prop valve.
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[treveiger]

where do I find a prop valve an mc? I looked for a prop but didnt have any success. Could I take it off a 73 or up chevy truck?
Thanks

[lolife99]

Buy parts for a 71-72 from your local parts store.

Or buy aftermarket parts.
ECE or Performance Online would be my first choice.
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[treveiger]

Hey thanks Keith I really owe you one!
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[lolife99]

Keep us posted on your progress.

[treveiger]

Will do! You wouldn't have a pic of the mc an prop valve would you?
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[FirstOwner69]

Originally Posted by treveiger

Will do! You wouldn't have a pic of the mc an prop valve would you?
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There is a picture of an original GM master cylinder, prop valve and power brake booster in this thread ( http://67-72chevytrucks.com/vboard/s...d.php?t=505976) in the last pic in post 3 and only the prop valve in the first pic of post 4. I believe the prop valve and M.C. were the same without power brakes. Perhaps someone can verify...

Rock Auto's website also has some pics of the M.C. but no prop valves.

[lolife99]

Member teeitup always has this stuff for sale.
His latest thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=514555
has a '72 C20 with power brakes.

[treveiger]

Has anyone tried those adjustable proportional valve? I found some of those an think they might be nice.
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[RichardJ]

It sounds like you don't have any rear brakes at all.
With that switch block and no proportion valve or combination valve you should have been locking the rear brakes up very easily. In rainy weather the truck should have been swapping ends.
When you replaced the front pads and lost the front brakes, that basically confirms you don't have rears.
I think you probably have air in both the front and rear lines. The rear shoes could also be way out of adjustment.

I would start with a new DISC brake master cylinder and build from there.
A disc brake master cylinder has one reservoir that is extra large. Usually the rear half of the MC is for the front disc brakes. On drum brakes the shoes themslves are adjusted to compensate for wear of the shoe lining. The amount of fluid in the reservoir stays nearly the same throughout the life of the shoes. As disc pads wear, the amount of fluid in the caliper behind the caliper piston increases. Without a larger reservoir for disc brakes, it can empty and you will push air into the lines.
I like the '73 up combination valve that you mentioned, better.




Some combination valves may require the metering valve pin to be pushed in for bleading and some require it to be pulled out.



When I put the disc brakes on my '67 I used all the brake lines, combination valve and MC from a '76. There is a connector for the rear brake line on the passenger frame rail just below the firewall or passenger floor pan. It's the same on disc/drum and drum brake trucks. I used all the lines from there forward. This mounts the combo valve on the cross-member below the radiator and out of sight.

[michael bustamante]

also when you switch to a disc master cyl and prop valve you will have to change your front hard lines also as theyre different thread, the rear line you can just get an adaptor

[treveiger]

I looked online and it seems the 71-79 brake mc are the same. The prop valve would have to work on a 71 brakes wouldnt it? Did the manual breaks have different prop valves than the power breaks? I have a power break set up with some kind of a valve but idk if itd work. It doesnt look like the one above. Its off a 71-72 gmc c15.

[stsalvage]

when it comes to brakes Best off buying new.I know times are tough but you don't really want to run down a Kid that's manslaughter and if your working on your brakes. Not a skilled shop.I'm a safety guy.When it comes to brakes I like having as many friends around me that can help me.I know there are some Old classic guys in your nick of the woods that would love to help ya.

On my 1965 i did a brake conversion and i used a 1972 3/4 ton But i used the Cast iron proportion valve there a little pricy but there good.Auto zone carries a boaster and master for 125.00 combo kit if you have the bracket for the booster and the rod from the old one your in luck

[stsalvage]

more pic's

[treveiger]

Originally Posted by stsalvage

more pic's

I'm planning on buying new manual mc, but I'm having trouble finding the prop valve new. I found a after market adjustable one but Idk if those work on our pickups. I think you only need one of them for adjusting the back brakes am I correct? That's why I figured if I got the prop valve from another pickup I could use that.
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[RichardJ]

]


This is what a proportion valves does. It allows for the same pressure front and rear up to about 500-600 psi. After that, the rear gets increasingly less pressure than the front brakes. Too much pressure to rear drum brakes on a Disc/Drum setup will lock the rear brakes up.

Very few production vehicles from that era have stand alone proportion valves and they are non-adjustable. These were used on 1st generation Camaro, Chevelles and the same one was used on B body Mopars.


Drum/Drum did not use a proportion valve, just a block for the switch.


The combination valves that I have in my other post were more typical and is what were used on these trucks.

Combination valve has three parts.
1. Metering valve - or hold-off valve - prevents any fluid from flowing to the front discs until the pressure reaches 40 psi. At 39 psi there is no pressure at the disc brakes, but the 39 psi on the rear drums will cause them to start slowing the car. The metering valve provents nose-dive when you start applying the brakes.
2. Brake switch - indicates brake failure. Same thing that you have on the truck now that was used on Drum/Drum brakes.
3. Proportion valve - As described above.

[ls1nova71]

Originally Posted by treveiger

I looked online and it seems the 71-79 brake mc are the same. The prop valve would have to work on a 71 brakes wouldnt it? Did the manual breaks have different prop valves than the power breaks? I have a power break set up with some kind of a valve but idk if itd work. It doesnt look like the one above. Its off a 71-72 gmc c15.

Prop valve from a '71-at least '79 will work. Manual brakes used the same prop valve, but a different master cylinder. 3/4 tons use a different booster and master than 1/2 tons, so if you have a 1/2 ton booster, get a 1/2 ton master. The booster in post #9 and the first truck in post #14 are 3/4 tons, the last booster pic in #14 is the 1/2 ton.

[treveiger]

Ok thanks guys this helps a lot!
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