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1970 Chevy C-10 brake problem

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ASEguy
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PostPosted: Sun Oct 11, 2020 5:19 am    Post subject: Re: 1970 Chevy C-10 brake problem Reply to specific post Reply with quote

A 1 1/8 inch bore seems too big if I remember right. That's a big if. The bigger bore would make it more difficult to stop and since your pedal seems normal sitting still, but you have difficulty stopping, that would be where I would start. I think you may be right with the new master cylinder being the culprit. I would stay with stock size and by pads that were more aggressive for the front since you have the 454. Let us know how this turns out.
 
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Determined
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PostPosted: Sun Oct 11, 2020 7:43 am    Post subject: Re: 1970 Chevy C-10 brake problem Reply to specific post Reply with quote

In no particular order

My thoughts and what would I do.

-Smaller bore master will marginally increase pressure, if the truck has single piston calipers the decrease in volume should not be an issue.

-Thoroughly manual bleeding of the system with someone else running the pedal for you will eliminate the chance of any air and it will give you a real good idea of what you have for flow and pressure at each wheel.

-Proper routing of lines to master cylinder and combination valve along with proper line size.
Front port is not always for front brakes.

-For the time it would take I would pull the drums, inspect and measure them, check everything over and adjust properly.
Glazed, oversized, contaminated linings, improper installation.

-Wheel cylinders can have the same casting number yet different sized bores be it from factory or from a rebuild.

-Does the wheel cylinder bore match what stock for the year would be?
Is one side of a cylinder stuck?

-Is there a residual valve for the drum brakes?

-Is the combination valve holding back the application of the drum brakes too much?
A big difference in a valve for a fiberglass body vette and one from a 9 foot longhorn.

-The setup you have if working properly would seem to be more than adequate to cause the front calipers to lock up solid if the rears were not functioning.

-Pedal travel, pushrod length and geometry.

-I have done a lot of brake conversions on equipment over the years and in general I would expect to see 700-900 PSI on manual brakes and 1100-1400 Psi on vac boot power brakes.

-9 out of 10 times when I resorted to checking actual line pressure the problem was not the sizing of the components it was either air, geometry, condition of components, restriction, blockage.

-A heavy foot should be able to stop that truck without any boost if everything is right.
 
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david from mo
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PostPosted: Sun Oct 11, 2020 12:20 pm    Post subject: Re: 1970 Chevy C-10 brake problem Reply to specific post Reply with quote

I had a some time so I checked Advance Auto site,Your pickup in 1970 had 1 inch master cylinder and 11 inch booster, that's where I start
 
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Hobo,NC
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PostPosted: Mon Oct 12, 2020 8:38 am    Post subject: Re: 1970 Chevy C-10 brake problem Reply to specific post Reply with quote


Looks like cheap tools will get you in trouble its a bad habit to use the vacuum gauge on a hand heal vacuum pump...

The master shows 14" of vacuum at idle and around 19 on decell so off we go to look into a electric vacuum pump set up...



https://www.streetmusclemag.com/tech-stories/gimme-a-boost-how-to-build-an-inexpensive-power-brake-vacuum-booster/

This still does not answer what its so hard to stop with no vacuum
I have a 1" master cylinder on order...
 
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Ark68SS
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PostPosted: Mon Oct 12, 2020 8:27 pm    Post subject: Re: 1970 Chevy C-10 brake problem Reply to specific post Reply with quote

Hobo,NC wrote:
This still does not answer what its so hard to stop with no vacuum
I have a 1" master cylinder on order...


Rear shoes installed backwards? You'll lose the servo effect if they are.
BillL
 
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Hobo,NC
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PostPosted: Sat Oct 17, 2020 4:38 pm    Post subject: Re: 1970 Chevy C-10 brake problem Reply to specific post Reply with quote


We have brakes if it had anymore I would have to take some off... Yeal Baby!

While waiting on the electric vacuum pump I took the rear brakes apart he told me it was all new. The shoes were new he did shine up and paint the wheel cylinders. The shoes were cooked the wheel cylinders were somewhat frozen up from rust. It got new shoes, wheel cylinders, drums, brake lines including the center rubber line, parking brake cables the old ones were sticking.

The linkage for the brake pedal and at the booster were way off I got that adjusted everything was just too long no slack in the system...

This is a slick pump it does the job...



https://www.amazon.com/gp/product/B0789PJWBN/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1




I believed him when he said it was all new I could see the rear shoes well thru the adjuster hole the fronts was a no brainier...









 
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