wont brake

[cardar911]

Hi guys
 I recently got my chevy c20 back on the road.. ive installed a ford 9inch rear end with new ford drum brakes new front pads and discs. I was getting bad pedal feel so we changed master cylinder and bled many times. Pedal feel is better now dont have enough braking power.. I cant get the fronts to lock up, just doesnt have the power.. we are out of ideas.. the booster seems healthy
thanks in advance guys
rob

[Irish_Alley]

hows the pedal feel? soft, hard, spongy or normal.
if you jack up the rear can the brakes stop the tires from spinning when in drive?
can you spin the front if you jack up the front and have someone step on the brakes?

[78BIG-TEN]

Assuming you have all the air bled out of the system and everything is alright as far as hydraulics,
I have seen the quality of brake pad material cause this problem.I had this on my 90 1500
tried several different brands of pads and finally was able to get it to stop well using Bendix
first line.With the others I could stand on the pedal with both feet and would only come to a gradual stop.
Something to think about

[jg1977c20]

front discs on older vehicles are designed to not lock up via the proportioning valve this was the earliest anti lock brake but only applies to the fronts - rear drums should lock up - in an emergency its your skill that will allow you to stop without locking up the rears .

[fxrsrider]

Rob,

The answers to Irish's questions are important for troubleshooting your problem.

Other questions....
Did you change your cam?
Are your vacuum lines and connections to the booster sound?
Did you bench bleed your new MC before installing it?

[78BIG-TEN]

The proportioning valve is just that,it limits the pressure to the rear brakes in relation to load\weight to keep the rears from locking prematurely.Some trucks will have another valve in the rear with a link tied to the rear axle for load compensation.If all is ok they should lock up.After all that's why they developed abs

[fxrsrider]

RE: "Proportioning Valves" w/regards to our rounded-line trucks.  The following is taken verbatim from the '74 manual (pg 5-7):

"All models have a combination valve. The combination
valve is located below the master cylinder on P models
(without Hydro-boost), on the crossmember on C-K
models and P models (with Hydro-boost) or on the frame
rail on G models. The front and rear hydraulic lines are
routed through this combination "metering" and "brake
failure warning switch" to their appropriate wheel
cylinders or caliper.
The metering portion of the combination valve tends to
"hold off" front hydraulic pressure until the rear brake
system overcomes their pull back springs; then pressure
is allowed to flow with the result being a good
distribution of braking effort.
The brake failure warning switch portion of the
combination valve "senses" a loss of hydraulic pressure,
if a failure should occur and turns "on" a red light in
the dash to warn the operator of the failure."

Re: Failure of the stock brake system (pg 5-2)...

"The combination valve (near the master
cylinder) detects the loss of pressure and illuminates the
brake alarm indicator light on the instrument panel. The
pressure loss is felt at the brake pedal by an apparent
lack of brakes for most of the brake travel and then,
when failed chamber is bottomed, the pedal will harden.
If a vehicle displays these symptoms, it is a good
indication that one of the systems contains air or has
failed, and it is necessary to bleed or repair the brakes."

The combination valve on a K is on the front end dropmember/crossmember.

Nevertheless, the initial problem doesn't seem like a proportioning valve or metering problem.  The problem is likely vacuum (indicated by the lack of "braking power").....except in the case that the pedal is, as Irish inquires, "spongy".....then there's likely air in the system.  Bleeding brakes is a pain-in-the-rear and done best with two people several times over.  As a result, most people don't bleed their brake systems properly.  And most DIYers don't realize that a "new" MC needs to be properly bench bled before install.