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-   -   Power brake system won't lock brakes (

Chief-e9 10-20-2011 05:03 AM

Power brake system won't lock brakes
I installed dual master cylinder and booster into my 47 stake-bed with 351W engine. I used a kit made to replace the original under the floor. (kit came from MBM brakes and is designed for a 1/2 ton) Thus,had to modify the 3-bolt pattern somewhat by elongating one of the holes plus cut and weld original push rod to the new push rod. Also, installed four 10 lb residual valves as well as a metering block to prevent front end from diving (couldn't really tell any difference with or without block but installed since I had it.) The pedal pressure is lighter now and is much like any vehicle with power brakes and it slows the truck very well. (I am still using the stock drum brakes.) The big problem I am having is when I push the brake pedal as hard as possible to "lock up the brakes" for a panic stop it doesn't happen. The truck will slow down fast like I have an anti-lock system but just won't lock em up. Still trying to figure out why. My mechanic said the wheel cylinders, pads, drums, etc. are good. Wondering if there can be a problem with the size of wheel cylinders as related to size of M/C, etc.? Any ideas and suggestions will be appreciated.

Bonneville462 10-20-2011 06:24 AM

The metering block is designed for cars with disc/drum set ups.. It slows the front disc brake action down so the mechanicals of the rear shoes can catch up and the brakes apply evenly..

I never heard of having a metering valve on a all drum set up.. That's not to say it can't be done though.. I would look into that..

Also, is the master cyl plunger all the way seated when the pedal is not in use..

I get a lot of good suspension and brake info here...

enjenjo 10-20-2011 12:25 PM

I suspect that the booster you have is a 7" single diaphram booster. Id so, a double diaphram booster will help. If there is room you can go to a 8" double diaphram booster, big difference.

There is also a difference in master cylinder bore between a F1 and the bigger trucks. You may have to change the master to match the wheel cylinders you have.

The metering block has to be a drum/drum metering block, not a drum/disc. With a dual circuit master cylinder you should use a metering block, it adds protection if one brake circuit fails.

The other thing to check is pedal ratio, you want a ratio of about 5 to one, or you will never be able to apply enough pressure with your foot to lock up the brakes.

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