1970 Chevy C-10 brake problem

[Hobo,NC]

I have a 1970 C-10 with a 454 automatic it has a lite cam it idles well no problem with that. It came to me with the complaint of no power brakes and has not acted like power brakes since he brought it. It has a small 8 brake booster mounted on a bracket under the hood that looks OEM, it has what looks like factory front disc and rear drums everything looks new except the rear brake drums. I am familiar on how to diagnose a bad booster when you start the engine with your foot on the brake the brake pedal sinks a little then you have a rock solid pedal. The booster has a good supply of vacuum at idle (1 I have removed the vacuum supply down to the port in the intake checked/replaced the vacuum line to the booster. I went as far as to supply vacuum with a new line out of the back of the carb throttle plate (3/8 port). The booster DOES hold vacuum after the engine is shut down a new check valve was added because I had one. I am confident the vacuum supply is good, supply line and check valve and the booster. There is no issue with the pedal and linkage I have a full rock solid brake pedal. The master cylinder is new I measure its an 1-1/8 bore all brakes do apply tho the rear is a little weak. It stops badly with what seems like little power access if you shut the engine off you may not stop it



I wanted to put a hydra boost on it, it has power steering the owner gave the command to replace the booster I ordered an 8 booster from Speedway dual diaphragm #91031485 its no better: ( At this time I am thinking try a 1 master cylinder.



What are your thoughts are suggestions.


 

[Hobo,NC]

Try a 1" bore master cylinder.
 

[steve_in_mo]

Brake proportioning valve?
 

[david from mo]

At Chrysler training school I was told of about the same story.On this car the fix was softer pads and shoes they were hard and glazed.
 

[TGP]

There's no part of that booster/master combo OEM
Not even sure front disk brakes are factory although it
Was around this time period they were coming online to trucks.

Go with the bigger master Cyl.

Tom
 

[Hobo,NC]

I have been typing asking myself questions the usual talking to myself when I am LEFT ALONE NO PRESSURE WINK WINK...

The brakes on this thing acts just like a low vacuum issue on a lopey cammed up jalopy... It does run out nice tho it has no more power than a run of the mill stock 350 it will not break rubber when you nail it from a rolling stop.. It did show 25" of vacuum on decell with a run of the mill hand held vacuum pump/tester... I would have thought I would have good power accessit just on one stroke of the brake pedal...

I am gonna back up and pull out a master vacuum tester...
 

[504]

Rear brake shoes on backwards. Rear brakes out of adjustment,rear parking brake
not installed right. Front brakes with organic pads instead of semi metallic
pads. I only see one brake line coming from the transfer block. Just throwing
things against the wall.
 

[Hobo,NC]

All I know is some 60's had this set up tho it may have had a 11" booster.. This is the same set up I had on a 63 chevy 1/2 ton I got it off of a GMC of that vintage early to mid 60's... It would stand the truck on its nose : ) I had to meter the vacuum to it to keep from trowing my beer in the floor..

On a hydra boost they offer firewall mount are to this mount...

Pads rotors brake shoes they look new I am not sold its a pad/shoe issue... I am still leaning to a power accesit issue...

Brake proportioning valve ?

Not sold on that either it will not slide a wheel any wheel on a rock drive... All wheels do have pressure tho the rears do not fully lock up on a lift close but no cigar...
 

[Hobo,NC]

[Hoenes]

Like to add, front brake lines can look perfect on the outside, but be deteriorated and collapsed on the inside.
 

[504]

I just thought of something else,is there an adjustable brake valve somewhere on
the rear brake line? Try stopping it without the booster hooked up. It will be
a hard pedal, but the brakes should still stop.I de expect you will have a
different master cylinder on it to make it stop. I just thought of something
else, isn't that a drum brake master cylinder? Maybe not,it's been awhile since
I worked on those older systems
 

[Steve@Advance]

If it has a high hard pedal, you can go with a smaller bore master cylinder, see what that does.

A bigger diameter booster would also help.

I had a 89 C1500 with an early booster, had terrible brakes. I changed it for the later booster, slightly bigger diameter and thicker (not sure what difference thicker made) but it sure had better brakes!
 

[guido]

Hello Hobo,NC

I think that you are getting plenty of
pressure. I don't think then, that a
bigger master cylinder will cure the
problem.the pressure may not be getting to
the wheel cylinders? You can rig a gauge
setup? My first thought is more likely a
brake material issue. The rear being drums
may have the shoes not correctly install.
You may have checked that already. As you
know the softer the shoes mayerial , the
better they stop, although service life is
a bit shorter.
With all that braking power, that thing
should stop in a dime?

Guido.
 

[8NChris]

[M-MAN]

If I remember correctly that was about
the time frame Chevy came out with the
equalizer valve. (Now a day's it's
called proportioning valve. ) I thought
it was a nightmare. They used to sell an
aftermarket connection block to get rid
of it. That's where I would look.