Need Help w/ Brakes

[rico7769]

Hello Everyone,
My bro and I have recently acquired our first 67 GMC. Its great to see the website support and all the info we have read from the loyal supporters!
Our dilemma is this, we recently replaced the master cylinder, both front wheel cylinders, cleaned and adjusted all parts and shoes, and replaced the flex rubber lines leading from the steel brake lines into the wheel cylinder. The brakes were bled several times on right and left front. The setup is dual reservior so we did not bother with bleeding the rear as the back reservior did not move when the bleeding was taking place. The setup is drums all the way around (manual, not power assited anything) w/ parking brake lever under the column. We have inconsistency in the pedal. After a few pumps, it seems to be hard as a rock with no travel. Once the truck is put in reverse, it rides down about an addt'l inch or so requiring me to press it again and it returns to high and hard. Brakes still seem very intermittent. I don't think pressure from the master is the issue, however I don't know! We have a steep driveway so I am leery of backing out too much. I read that the brakes have to "self adjust" by driving in reverse at about 10mph and engaging the brakes. I also read that the emergency brake may have to be engaged for the self adjusters to activate.

I can't figure out why the pedal is inconsistent and the truck is not willing to react no matter how quickly or gently I press the pedal.

Any help??

[old Chevy guy]

The front brakes won't be affected by the parking brake at all and I never heard of having to engage the parking brake to get the self adjusters to work.If the brakes were manualy adjusted when you installed them they should not need to be adjusted by the self adjusters to work properly.Did you adjust the shoes with the drums on?Don't be afraid to make them a little tight .Were the drums cut?I would check the back brakes to make sure they are working properly too,just because the back fluid didn't go down when you bled the front dosent mean the back couldn't have trapped air.Also maybe they need to be adjusted up too. The rear wheel cylinders could be frozen and one or both may not work at all. When you pump the brakes the pedal will get hard as the brakes travels out but will fade back if the shoes are not adjusted enough or if they need more bleeding.

[rico7769]

I adjusted them with the drums on while doing the front and haven't paid much attention to the rear yet. I'll start by adjusting the rear and checking the cylinders for "crud". (The front ones had to be original to the truck). Then we'll bleed the rear and take her out for a run. May even replace the rear cylinders while we're there. It only took an hour or so to do the front last time. Hopefully this will cure the issue. We've been working overtime on the motor for so long that we have neglected the crucial power of STOPPING!!! By the way, do you know how much horsepower a 72 LT1 made stock? Just curious.

Thanks for the quick reply, too.

[El Jay]

It could be your proportioning valve.
Check out this link & maybe it will help.

Good luck!

http://www.classicperform.com/TechBo...t.htm#testprop

[rico7769]

Will give it a shot over the weekend. I think incorrect bleeding order is the culprit. We'll find out on Sat. Thanks for all the info and I'll let you all know when we finish!