Brakes - am I expecting too much
 

I have a 67 2wd. The brakes are all 100% new either by me or the previous owner. The front is a CPP disc kit, including booster and master cyl, and the rears are rebuilt drum brakes.

My issue is poor braking. The front brakes don't seem to do anything. My latest experiment was to completely disconnect the rear brakes. I drove it and it will come to a stop, but no matter how hard I push the fronts will not lock up. I can drive in reverse and slam on the brakes and the fronts will lock up. I know trucks are light in the rear end, but any type of panic push on the pedal results in immediate rear lock up. I replaced the combination valve thinking it was bad, but no change. I've bled and rebled the brakes at least 1/2 dozen times. I added a wilwood proportioning valve to the rear and have it adjusted to minimum (50% output). This makes the truck drivable. But still not good. I have had dozens of vehicles with this same gm brake set up and never felt the brakes were scary like these are. Any ideas on what's happening?

Re: Brakes - am I expecting too much
 

How are the lines from the master routed? IIRC, 1/2T uses the front reservoir for front brakes, and 3/4T and 1T use the rear reservoir for the front brakes. Can you post a pic of your setup?

Re: Brakes - am I expecting too much
 

What Steevee said, particularly since you've added a second prop valve and it made a difference.

Other thing I'd want to verify is that the pedal rod distance to the MC is correct.

Assuming you have a hard pedal from a bled system of course

Re: Brakes - am I expecting too much
 

I'm actually facing somewhat the same issue, with basically the identical setup. I was having these issues and the brake booster had failed, so I bought the nice booster/master/prop valve setup from CPP.

Everything seems to be working correctly; I have a high (2.75"), hard pedal and I've gone though all the CPP troubleshooting. When CPP ships the master/booster together, they set the rod length. But it feels like the rear drums are doing all the work, although they're not locking.

I'm planning on replacing the front calipers just so I can say it's "all new" and then chasing it back from there. I bought the Motive adapter so I can use my power bleeder and ensure I've got good flow.

I don't mean to threadjack, but just to note that I'm having a very similar issue and will share my troubleshooting as I go along to help the OP...and maybe see it get solved by them and take advantage of that experience :)

Re: Brakes - am I expecting too much
 

Well assuming plumbing is right, a front disc C10 should be able to lock up brakes. On the 72 I had it was a disc drum manual. I put on a CPP power booster/MC when I did a complete brake job as the brakes were original to the truck. New calipers, soft lines, drums, shoes, wheel cylinders for rear. brakes would throw you through the windshield.

OP, confirm your MC/PV is plumbed right for the front disc to MC

Re: Brakes - am I expecting too much
 

I was disappointed with my rebuilt brake system as well. It's a '72 K10 with 13" rear shoes on a 14 bolt, and 12.50" discs in front. I used a hydroboost from a 2005 Silverado, and connected the pedal rod directly. The factory '72 vacuum assist used a bellcrank with essentially a 2nd pivot point that increased the pedal ratio.

Keep in mind that the proportioning valve will "shuttle" the center valve blocking fluid to the front or rear. Mine was "shuttled" closing off the front. After bleed the rears, I slammed the pedal a couple of times until the brake warning light turned off. Then I could bleed the fronts.

Also, the brake assist rod needs to have proper gap to the master cylinder.

Re: Brakes - am I expecting too much
 

When you bled the brakes did you use a proportioning valve bleeder tool?
Basically it centers the valve for bleeding to insure a good even brake bleeding
front and rear.

https://www.summitracing.com/parts/s...iABEgJNVfD_BwE

Re: Brakes - am I expecting too much
 

Without getting into the super long story, I'll give more info.

I bought the truck as an unfinished project with the disc brake kit installed. I got it running and drove about 1000 miles. Was not happy with brakes. I checked the rear brakes and noticed they were not working. Rebuilt them with all new parts and a new rubber hose. This is when I noticed that the brakes were always on. Further investigation showed the master cyl to booster clearance was <0. I got that adjusted. Also the combination valve was the old one from drum/drum system. Ah ha I replaced the combination valve and no change. Then I decided that the front brakes were probably glazed due to dragging all the time. Got new pads and also sanded the rotors heavily to remove any glazing. Still no change. Just in case, I got a new master cyl from the parts store. Installed it and no change. Everything is setup up correctly and I always use the tool in the combination valve when bleeding.

At this point my plans are:
1) Replace rear brake hose in case the new one I bought is collapsing. (but that won't fix the front brakes)
2) Get a brake pressure gauge and check to see that the calipers and rotors are getting proper differentiated pressure. If I have proper pressure, I guess I will replace my brand new calipers and brand new rotors....lol.

Re: Brakes - am I expecting too much
 

Just to be clear. The tool will not center the valve if tripped. It will only hold an untripped valve in place so as not to slide and block the front or rear circuit. If you use the tool and can't get the rears to bleed correctly, it is a failed proportioning valve on the back of the PV combo valve and you will need to replace. New ones are more often than should be DOA.

Re: Brakes - am I expecting too much
 

PbFut, Yes I am familiar with the operations of the combination valve and I am suspecting that I got a new defective one. It may be a disc/disc mislabeled as disc/drum or may just be bad.
I just pulled the metering section off the front and found an O ring that was "rolled" a little which may be my problem. Will report back when I get this figured out.

Re: Brakes - am I expecting too much
 

And I didnt say otherwise. That is the reason why I asked the question. But if he did use the tool then thats not an issue for him. If he didnt then he'll want to center or reset the valve and then install the tool. Now he can bleed the brakes again.

Re: Brakes - am I expecting too much
 

Another thing to consider. How much vacuum are you getting to the booster? You should have a dedicated vacuum line going to the booster and nothing "T'd" into it. If you have a camshaft with a lot of overlap and have low vacuum, that will drastically affect your brakes.

Gary

Re: Brakes - am I expecting too much
 

cj847 wrote,
>>It may be a disc/disc mislabeled as disc/drum or may just be bad. I just pulled the metering section off the front and found an O ring that was "rolled" a little which may be my problem.<<

The Metering Valve will prevent nose-dive at low pressures. You didn't mention that as a problem.

Metering Valves have a threshold pressure of around 50-100 PSI. Say you have a Metering Valve with a set pressure of 75 psi. It will only restrict pressure to the front brakes at any pressure below 75 PSI.
Set your foot on the brake pedal and say you are generating 50 PSI in the hydraulic system. The pressure is overcoming the drum shoe springs in the rear and starting to drag the brakes. The front calipers are not yet working and you have NO nose-dive.

Push a little harder on the pedal and generate 200 PSI. The Metering Valve is OPEN and out of the picture. The Proportion Valve may have a minimum threshold pressure of 400 PSI. At 200 PSI, The front calipers are getting 200 PSI and the rear Drums are getting 200 PSI. The Metering Valve, the Pressure Switch and the Proportioning Valve are completely out of the picture.

One of the aftermarket brake suppliers mention that the difference in their disc/disc and disc/drum combination valves is that they simply remove the Metering Valve and insert a plug in its place. They will even sell that plug if you want to convert one yourself.

You may have rolled the O-ring when removing the Metering Valve.

1st pic, Freestanding Metering Valve used with GM disc brake systems in late "60s.

2nd pic, Freestanding Metering Valve from the aftermarket.

3rd pic, Metering Valve internals for replacement.

Note: the Metering Valve does not touch, is not part of and in no way does its function have anything whatsoever to do with the Pressure Differential Switch that happens to be also located inside the Combination Valve.

Re: Brakes - am I expecting too much
 

>> did you use a proportioning valve bleeder tool?
Basically it centers the valve for bleeding to insure a good even brake bleeding
front and rear.

https://www.summitracing.com/parts/s...iABEgJNVfD_BwE<<

Not only would I never use one of those, I am hard pressed to even call that a tool.

I had a great laugh some years ago watching Roadkill Garage. David Freiburger and Steve Dulcich were bleeding the brakes on some heap so they could do the only thing they know how to do with a car and that is to destroy tires for absolutely no reason.

Freiburger was pumping and Dulcich was cracking the valves. Not long into their venture, they snapped the nipple off the bottom of their plastic THING.

If you are vacuum bleeding, you're using 20" hg to move the fluid and air. If you are pressure bleeding, you are using about 15 PSI.
When manually bleeding and you have part of the system bled, you can be generating several 100 PSI, depending on hard you push at the bottom of the stroke.
That plastic nipple doesn't have a chance.

Freiburger fished the broken piece out with tweezers and used the tweezers to push the piston back to center.

When I finish bleeding, I crack a valve on the opposite end and use pressure to gently push the piston back to center.

Re: Brakes - am I expecting too much
 

Brakes can be a *****. Replace everything! Check pressure at the calipers and rear cylinders. Trace the system until you find the problem. Electric vac pumps help, if engine vac is low. If it is a 1/2T, my brake systems are 30% more powerful than original, but everything else has to be right.

Re: Brakes - am I expecting too much
 

https://67-72chevytrucks.com/vboard/...d.php?t=779896

I don't get most of this computer stuff, so maybe this will help...if it transferred OK.

Re: Brakes - am I expecting too much
 

Did you change the pedal ratio?
Or even check it!

Re: Brakes - am I expecting too much
 

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My understanding with the ratio of the pedal is that upon conversion from manual to power brakes, if the ratio isn't changed (by either moving the pivot point on the pedal arm or adjusting/adding the bell crank mech behind the booster), the resulting brake feel will be "strange" at best and inconvenient at worst but not represent a safety issue.

I'm currently dealing with some brake issues myself and am interested to get your opinion in this regard. I have no bell crank behind the booster and am currently using the stock, drum/manual brake pedal arm linkage hole. They work good but smooth operation is not as easy as all the other vehicles I've driven, I think, due to the ratio thing.

-Kevin

Re: Brakes - am I expecting too much
 

It don’t cost much to drill another hole.

Re: Brakes - am I expecting too much
 

I would think that the ratio with manual drum brakes to disc would cause the brakes to be to much since manual uses a higher ratio to apply more pressure to the master . Opposite of what the OP is experiencing
I converted my 68 from manual to power disc used CCP parts with a booster from Pickmup I have to say they are a little more aggressive than the factory power disc on my 72 . The booster has 30% more power so I attribute it to that more than the ratio

Re: Brakes - am I expecting too much
 

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Agreed. However at the time, when I was under there contemplating this, I found no easy way to drill that hole without a 90 degree drill head and I didn't want to disassemble/remove it to the bench either as that didn't look fun.

I'll revisit it soon though as my new booster is making strange noises and hissing sounds.

-Kevin

Re: Brakes - am I expecting too much
 

Hissing noise? The booster will hiss when you apply the brakes but should make no noise when you aren’t on the brakes sounds like a bad diaphragm.

Re: Brakes - am I expecting too much
 

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It hisses or makes a strange hissing type sound only after initially putting pressure on the pedal to slow down and then releasing slightly. That release motion causes the noise and only while the pedal is moving toward me.

It's also starting making a single bird-like chirp at the bottom of my braking arc of the pedal, that is, at furthest point down toward floor I push it and stop then begin releasing. Right at the bottom, sometimes it will chirp. Note I'm referring to the pedal stopping motion rather than truck coming to stop.

-Kevin

Re: Brakes - am I expecting too much
 

Technically, the front brakes should not lock up or you would loose that ability to steer.

Did you bench bleed the master cylinder in a vise?
Is the booster working?
Are your caliper pistons and wheel cylinders retracting? If they are not, fluid still returns to the master cylinder and each time you press the brakes those areas need to fill back up before your going to get a pedal.

Fluids by nature do not compress, its the mechanical linkage ratios that transmit leg to stopping power

Re: Brakes - am I expecting too much
 

>>Technically, the front brakes should not lock up or you would loose that ability to steer.<<

Better the fronts than the back brakes. When the rear brakes lockup, the vehicle will swap ends, so you better have clear lanes on both sides of you.