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Old 11-14-2017, 11:14 PM   #1
67chevy_hotrod
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Very strange brake dilemma!

Getting sort of desperate boys, I need my truck to be back to 100%.. no back brake pressure at all and very soft pedal. The fronts will work but only when my foot is buried in the pedal. All new brake lines, everything is new. When bleeding you get loads of brake fluid. It's all been bleed, multiple times there is no air in it. What I am understanding is I have the volume but not the pressure.

My truck is a 1969 3/4 ton Longhorn. I put performance online front disc brakes on it. It's got a 1972 GMC Dana 60 rear drum. In THEORY(I realize aftermarket parts aren't stock), all this is essentially stock 1971/1972 equipment. Now I've gone to Napa and picked up a 1969 3/4ton booster and master cylinder and installed. For a proportioning valve, I used a part identical to the one POL sent me, the new one is from RPC. My first question is will the pre '71 ( I say this year cause it's first year of disc brakes) booster/mc setup work with my current brake setup? Is the input rod the same length? My second question is what reservoir goes to what brakes on the 3/4 ton MC? I understand it's opposite of a 1/2 ton? Is that difference in front to back in the MC or proportioning valve? Currently I have front reservoir going to front on prop . valve and rear to rear. Is that backwards? Any help is appreciated, the truck is not safe to drive right now and I need it. Thanks again!
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1972 Chevy Cheyenne Super 10 "Sherry"
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Old 11-14-2017, 11:23 PM   #2
bigrigpardo
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Re: Very strange brake dilemma!

ok was your truck factory power brakes before? do you have the brackets between the firewall and booster. a drum drum master wont work right with disc. now that being said it shouldn't give you no rear brakes but may drag the fronts. on a disc drum master the rear reservoir is the front brakes and front the rear. the rods between booster and master are different. there is a short rod and long one depending on what booster if I recall right. but pull your master off the booster. there is a long and short rod. one goes into the master about an inch and one just goes to the edge. out some pics of your set up up so we can see if anything is obvious.
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Old 11-14-2017, 11:25 PM   #3
jocko
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Re: Very strange brake dilemma!

69 mc is for a 4xdrum system, you kinda need a 71 or newer C20 mc (the C20 or heavier is important as the mc's are different between 10 and 20 - and note they are plumbed differently. In a C20, if I recall correctly, the front reservoir is for the rears and the rear reservoir is for the fronts). Also - since you put new lines, did you step down to 3/16 lines for the front disks from the 1/4 for drums? It's not absolutely required, but that is the factory arrangement for disk/drum. Your 69 booster should be fine. Did you have power brakes when it was all drums? Those are the first things that come to mind. Is your prop valve adjustable?

EDIT: Oops! Sorry Bigrig you posted much of this - I was still typing...

Regarding the rod between the booster and master - the master needs to be for power brakes - and it should be short regardless of year because the hole in the piston in the back of the mc is shallow. Manual brakes have the deep rear hole if I remember. Also, I thought (it's been awhile) that the front/rear reservoir difference existed between C10 and C20/+, not necessarily between 4x drum and disk/drum systems. Guess for this question it doesn't matter - no matter what, your front reservoir should be plumbed to the rear brakes and rear reservoir to the front - like we've both stated.

Last edited by jocko; 11-14-2017 at 11:31 PM.
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Old 11-14-2017, 11:37 PM   #4
67chevy_hotrod
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Re: Very strange brake dilemma!

Big rig and Jocko, the truck was no power brakes before. I got a 3/4 ton power brake setup from POL (since been replaced by my Napa parts) but I retained the bracket it came with. Non adjustable prop. valve. It's stock brake line size for a '69. So what I am gathering is I need a 71/72 3/4ton booster and MC. Rear reservoir is front brakes. And front reservoir is rear brakes?
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Old 11-14-2017, 11:42 PM   #5
bigrigpardo
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Re: Very strange brake dilemma!

yup. forget the pol crap for booster and master. go on the parts board and find an original 3/4 bracket setup with booster and stock prop valve. I have hear lots of issues with repros leaking. I have a cpp one in one truck and its leaking.then buy a new master 3/4 disc. yes front is rear rear is front. bigger res is for the disc.
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Old 11-15-2017, 12:00 AM   #6
67chevy_hotrod
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Re: Very strange brake dilemma!

Quote:
Originally Posted by bigrigpardo View Post
yup. forget the pol crap for booster and master. go on the parts board and find an original 3/4 bracket setup with booster and stock prop valve. I have hear lots of issues with repros leaking. I have a cpp one in one truck and its leaking.then buy a new master 3/4 disc. yes front is rear rear is front. bigger res is for the disc.
Rock n Roll! Thanks boys I pray this gets my truck back on the road!
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Old 11-15-2017, 04:52 AM   #7
RichardJ
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Re: Very strange brake dilemma!

The real difference between a MC for power brakes and a MC for manual brakes is the hole on the end of the piston. Shallow hole is for power booster, deep hole is for manual brake rod.
CPP for any given bore diameter, sells the same MC and includes an adapter pin insert.
Piston bore diameter differences won't stop brakes from working. A larger bore requires greater foot pressure, so is generally used with a power booster. Smaller diameters require less foot pressure and are used with manual brakes.
The pin on the front of the booster is adjustable and must be checked for proper length before mounted to MC. There are checking tools made, but if you are careful you can check by making measurements.
Using the larger reservoir for disc calipers is done so that it doesn't run out of fluid before the disc pads wear out. You'll have brakes with the small reservoir, but you better keep a close eye on the fluid level.
The factory used a COMBINATION VALVE.
One end of the COMBINATION VALVE has a proportioning valve and is plumbed to the rear drums.
The other end of the COMBINATION VALVE has a Metering Valve and is plumbed to the front disc calipers. The Metering Valve is identified by a pin that must be pulled out or pushed in, depending on the type, when bleeding.
If you have the brass type combo valve the Metering valve pin is pulled out when bleeding for most of them.
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