New Brakes, It would be nice if they worked, 67 GMC

[buds56]

On my 67 GMC 2wd, original manual drums
I installed an entirely new brake system

10 bolt rear out of an 82 chev tk, New CPP front disc calipers,hoses, rotors

New rear drums, shoes, wheel cylinders ,hardware, rubber hose, hard lines, 1/4" rear, 3/16" front,

Bought a new large Master cylinder, booster, prop valve kit for a 67-72 truck, it came with a firewall mounting bracket off ebay.


It had a fairly hard pedal and just wouldn't stop very well, the pedal would go down halfway and that was it, would just slow down and wouldn't pump up.

Changed the master cylinder, no difference, The booster quit working completely so I bought this setup,

https://www.summitracing.com/parts/sum-760197

Benchbled the cylinder and the pedal is a bit better, not great and the power assist is somewhat working, but it still just slows down, will not lock them up in gravel.
Will push fluid out the bleeders when pedal is depressed

On my last truck (61 chevy) I had to drill a new hole in the brake pedal to correct the ratio for the new power brake set up.

This has the linkage on the bracket between the pedal & booster and I don't see any info on pedal ratio for the 67-72's

I've had alot of these trucks in the past ( mostly built from junk)and never this much trouble or expense.

Any ideas would be appreciated

Thanks

[Mike C]

I’d start by inspecting the rear brakes again. Making sure the pads are appropriate for the drum size (diameter and width) and that they are adjusted out as far as they will go. Check for leaks at all 4 corners as well as the center line in the rear. After replacing all the other parts, you may want to look at the combination valve as being bad.

[03BlkZ]

Ok this may sound really stupid but it happened to me with my CPP kit on my 70. I am an aircraft mechanic and have pretty good mechanical ability and I still screwed this up. Check to make sure your calipers are installed on the correct side. The bleeder nipple needs to be pointing up. I had them backwards and they were pointing down. I fought this for a couple of weeks beating my head against the wall before I realized what I did. Swapped sides and the brakes were perfect. Hope this helps.

[68PanelTruck]

Just throwing 2 things out there:
1) Did you set the pin depth on the new master cylinder? The rod (pin) that comes out of the brake booster and goes into the MC has to be set with the proper gap. Too much gap and you will not get the full travel of the piston. Too little and your brakes can drag and eventually lock up. Youtube has a few good videos.

2) Does the proportioning valve need to be reset? This actually happened to me. If you start with a completely new empty system, the valve in the prop valve can shift off center (forward or backward) and completely block off half, front or rear, of the brake system. This is a safety feature so that if you get a leak, it shuts down that half of the system and you can still make an emergency stop. There is a little tool that you temporarly screw in the prop valve in place of the brake switch that holds the valve centered while bleeding the brakes. You can test the switch continuity with a meter by touching the little post in the switch with one lead and grounding the other lead. You should NOT have continuity if the valve is properly centered. If it shows continuity then you need to recenter the valve.
This is the switch that triggers the brake light on your dash. I used an ice pick type tool in the hole where the switch goes to pry the valve back to center. You can then bleed the brakes but this can happen again unless you use the tool to keep it centered.

[HO455]

Here are some links that may help.

http://67-72chevytrucks.com/vboard/s...d.php?t=801705

http://67-72chevytrucks.com/vboard/s...d.php?t=470102

http://67-72chevytrucks.com/vboard/s...d.php?t=644119

[buds56]

Today I opened all the bleeders for about 45 minutes keeping the master cylinder full, went through over a quart of brake fluid.

I then found a fellow to push the pedal for me and bled them, no air
(usually I'm by myself and use a jar and a hose to bleed each cylinder)

Checked the prop valve with a test light, it's ok
It has a full pedal, doesn't pump up

It stops ok, not great, I think it should lock the tires up in a panic stop.
I had a mid 80's Chevy that was similar in the braking dept. never liked it

When the seat comes back from upholstery I'll be able to put some more miles on it, If I don't see any improvement,
may put the rear drums on a brake lathe for a minor fast cut which should help the rear shoes seat better

Thanks for the help, Bud

[geezer#99]

Stock 67pedal ratio is 6 to 1.
You need 4.5 to 1 for power.
Usually a new hole about 1 inch lower gets it done.

How much vacuum does your motor produce at idle.

[buds56]

Today I pulled the brake pedal and drilled another hole 1-1/8" lower.

That did it, now the tires lock up.

I assumed the linkage on the back of the booster would compensate or correct the difference in ratio needed, but apparently it didn't in my case.

Thanks for the help, Bud

[HO455]

Excellent!

[Richard]

Awesome it worked out! Thanks to Geezer#99 for the suggestion.

[RustyPile]

Originally Posted by buds56

Today I pulled the brake pedal and drilled another hole 1-1/8" lower.

That did it, now the tires lock up.

I assumed the linkage on the back of the booster would compensate or correct the difference in ratio needed, but apparently it didn't in my case.

Thanks for the help, Bud

Not sure why you had to drill another hole.. Maybe because of the year of your truck.. I didn't have to drill a hole when about 3 years ago, I converted my '71 to power brakes.. In the process I replaced all calipers, wheel cylinders, pads, shoes, drums, rotors, hoses and steel lines.. I installed the same booster, master cylinder you did.. It took a while to get everything bled, but my brakes will put you through the windshield in a panic stop..

[gmc 2500 4x4]

nice thread, im having same issue as my truck has been apart for 10 yrs, (69 gmc k2500) all new except that proportion valve, drums all around. seems like i need to adjust the rod travel. i assume i need a "drum drum" master cyl?

[MikeB]

Glad it worked out for you. I have found aftermarket kits to be hit or miss. Boosters can be too small, M/C bore the wrong size, and a generic one-size-fits-all proportioning valve that assumes your truck has the same weight distribution as a Nova.

My 69 C10 has a factory stock M/C, booster, and prop valve from a 71-72 truck, along with spindles, disc brakes, and steering linkage from a mid-80s truck. (Center link and tie rod ends are larger diameter than stock 67-69 C10.) It has been working very well since 1995. In fact it almost works too well, and I may some day drill a lower hole on the pedal to make it a little less touchy.