Power brakes reduction in stopping power after motor swap, brake linkage swap

Finally got the new 451 running in the '68 Satellite. Just took an easy slow drive around the back lot to get a feel for the brakes. Only problem is, now the stopping power is not as good as it was when the car was running with the small block. Before the brakes felt great and the car stopped well. This is at 15-20 mph doing a panic stop just to check things out.

BEFORE:
360 ci motor (no vacuum measurement taken)
904 auto
Aftermarket brake pedal linkage with aftermarket 8" dual diaphragm booster and adjustable push booster push rod fully retracted for minimal MC engagement
Adjustable proportioning valve present, incorrectly installed between MC and brass distribution valve, but adj prop valve dialed all the way back to zero (effectively providing approximately 57% reduction in a specified pressure range, i.e. for panic stops)
Stock firewall reinforcement backing plate missing
Brake booster vacuum hose running to back of Edelbrock carb


AFTER:
451 ci motor with mild cam, measured 14 inches vacuum
727 big block auto
Stock brake pedal linkage (pedal now sits lower to floor) with aftermarket 8" dual diaphragm booster and adjustable push booster push rod adjusted to protrude approximately 1" adjusted for correct MC engagement
Adjustable proportioning valve no longer present, only brass distribution block for disc/drum
Stock firewall reinforcement backing plate
Brake booster vacuum hose running to intake manifold fitting behind carb

SAME FOR BOTH:
Same Raybestos 15/16" bore disc/drum Master Cylinder
Same wilwood forged dynalite front disc calipers
same stock rear drums
same vacuum brake booster

RESULT:
Brake pedal now sits lower, half as high as before
Brake pedal feels slightly softer
Braking power seems reduced. On a panic stop at 15-20mph, brakes were not grabbing hard and car seemed to take too long to come to a complete stop. If I didn't know any better, I might even think only the rears are doing all the work...

Any ideas as to what may be going on? Does my new motor's increased displacement have anything to do with it? How about the booster hose connection location change from carb to intake??

Bump

No one has any ideas??

Any idea How much engine vacuum you had before?

The booster hose was at the base of the carb before and is now on a runner? A runner should have enough but try putting back to the base of the carb and see if it makes a difference.

14" should be enough vacuum as long as the brake booster is good. See if it holds vacuum. Also with the stock linkage you might have less pedal force than before.

Unfortunately, no idea what engine vacuum was before with the small block.
Pedal feel and travel are one thing, but I can live with it.
Stopping power is another! I'm very surprised how different it is with all the same calipers, MC, etc.

The booster is basically new from master power brakes with 1000 miles on it. Worked fine before we pulled the motor.

Maybe there is another adjustment I need to do somehow due to decreased pedal travel at the adjustable booster push-rod end? Don't see how that could matter though. Maybe now I need to go to a 7/8" MC bore?

I guess I could also jack up the front end and spin the wheels and mash the brake pedal and see if the fronts stop well with no weight behind them?

Hey Dwayne:

My guess is you might be bottoming out your pedal travel with that small master cylinder. This happened to me on a factory power setup with a 15/16" bore MC. I ended up trying 1" and 1.031" and settled on 1" because it worked and required less force. Since you are describing less effort at the pedal, but reduced stopping power, this is my guess- your pedal isn't allowed to go any lower, and you're not building enough pressure in the master because the piston hasn't moved far enough.

One way to test is to swap the old linkage, but honestly a 1.031" master cylinder is cheap and may actually be easier to test with!

Put the prop valve back in. I have 14" too and mine works fine. Adjustable push rod from pedal linkage to booster?

From what was described, it sounds like there is just not enough pressure at the brakes (front or rear). There isn't a need for a proportioning valve if you can't lock up the rear tires. We need to figure out why there isn't enough pressure in the system and work from there. I'm sticking with not enough pedal travel due to the small master cylinder bore.

bigmanjbmopar said:

Put the prop valve back in. I have 14" too and mine works fine. Adjustable push rod from pedal linkage to booster?

Click to expand...

adjustable push rod at booster that engages master cylinder. It is adjusted to within 1/8". No adjustable push rod from pedal linkage to booster.

Why would putting the prop valve back in help with stopping power? I have read and heard from several people that with the stock brass disc/drum distribution block, that a separate proportioning valve is not needed...

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Need to try different size MC bore and if that doesn't work, change the linkage back. Thanks all!

as long as you are sure you have a prop valve(used in drum disc setups)and not a diverter block(used in all drum set ups)then it sounds like the master is too small.or you dont have enough rod travel before pedal bottoms out.are you sure the rears are not locking up?you said it felt like they were doing all the work.

I have to go along with the opinion that it seems to be a pedal travel issue. Let us know what happens and good luck.